Builder-vite: Replace .at() call with [] in codegen

[Chudesnov]

Unblocks loading Storybook instances built with Vite on devices locked to pre-2022 browser versions.

Closes #29030

What I did

Replaced usage of an API potentially unavailable in legacy browsers (Array.prototype.at) with a functionally equivalent alternative ([]). This should allow running Storybook in browsers that support ES Modules (script type="module") but don't have the Array.prototype.at method.

Checklist for Contributors

Testing

The changes in this PR are covered in the following automated tests:

• stories
• unit tests
• integration tests
• end-to-end tests

Manual testing

1. Run a sandbox for template, e.g. yarn task --task sandbox --start-from auto --template react-vite/default-ts
2. Open Storybook in your browser without Array.prototype.at support but with ES modules support (e.g. iOS Safari 15.3)
3. Access any story

Documentation

• Add or update documentation reflecting your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Checklist for Maintainers

• When this PR is ready for testing, make sure to add ci:normal, ci:merged or ci:daily GH label to it to run a specific set of sandboxes. The particular set of sandboxes can be found in code/lib/cli/src/sandbox-templates.ts

• Make sure this PR contains one of the labels below:

  Available labels

  • bug: Internal changes that fixes incorrect behavior.
  • maintenance: User-facing maintenance tasks.
  • dependencies: Upgrading (sometimes downgrading) dependencies.
  • build: Internal-facing build tooling & test updates. Will not show up in release changelog.
  • cleanup: Minor cleanup style change. Will not show up in release changelog.
  • documentation: Documentation only changes. Will not show up in release changelog.
  • feature request: Introducing a new feature.
  • BREAKING CHANGE: Changes that break compatibility in some way with current major version.
  • other: Changes that don't fit in the above categories.

🦋 Canary release

This PR does not have a canary release associated. You can request a canary release of this pull request by mentioning the @storybookjs/core team here.

core team members can create a canary release here or locally with gh workflow run --repo storybookjs/storybook canary-release-pr.yml --field pr=<PR_NUMBER>

name	before	after	diff	z	%
createSize	0 B	0 B	0 B	-	-
generateSize	77.5 MB	77.5 MB	100 B	3.74	0%
initSize	162 MB	162 MB	117 B	0.39	0%
diffSize	85 MB	85 MB	17 B	-0.45	0%
buildSize	7.57 MB	7.57 MB	-36 B	-0.43	0%
buildSbAddonsSize	1.66 MB	1.66 MB	0 B	-0.42	0%
buildSbCommonSize	195 kB	195 kB	0 B	-	0%
buildSbManagerSize	2.34 MB	2.34 MB	0 B	-	0%
buildSbPreviewSize	352 kB	352 kB	0 B	-	0%
buildStaticSize	0 B	0 B	0 B	-	-
buildPrebuildSize	4.55 MB	4.55 MB	0 B	-0.42	0%
buildPreviewSize	3.02 MB	3.02 MB	-36 B	-0.45	0%
testBuildSize	0 B	0 B	0 B	-	-
testBuildSbAddonsSize	0 B	0 B	0 B	-	-
testBuildSbCommonSize	0 B	0 B	0 B	-	-
testBuildSbManagerSize	0 B	0 B	0 B	-	-
testBuildSbPreviewSize	0 B	0 B	0 B	-	-
testBuildStaticSize	0 B	0 B	0 B	-	-
testBuildPrebuildSize	0 B	0 B	0 B	-	-
testBuildPreviewSize	0 B	0 B	0 B	-	-

name	before	after	diff	z	%
createTime	17.5s	5.8s	-11s -657ms	-1.24	🔰-198.6%
generateTime	22.8s	19.6s	-3s -196ms	-0.59	-16.2%
initTime	14.6s	14.9s	317ms	-0.93	2.1%
buildTime	9.9s	11.6s	1.7s	0.4	14.9%
testBuildTime	0ms	0ms	0ms	-	-
devPreviewResponsive	6.6s	7.5s	901ms	0.73	12%
devManagerResponsive	4.4s	4.8s	389ms	0.5	8.1%
devManagerHeaderVisible	782ms	1s	231ms	2.89	🔺22.8%
devManagerIndexVisible	817ms	1s	231ms	2.59	🔺22%
devStoryVisibleUncached	1.3s	1.5s	251ms	0.78	15.9%
devStoryVisible	816ms	1s	234ms	2.62	🔺22.3%
devAutodocsVisible	632ms	853ms	221ms	1.17	25.9%
devMDXVisible	666ms	842ms	176ms	1.56	🔺20.9%
buildManagerHeaderVisible	716ms	754ms	38ms	-0.39	5%
buildManagerIndexVisible	722ms	763ms	41ms	-0.5	5.4%
buildStoryVisible	782ms	844ms	62ms	-0.03	7.3%
buildAutodocsVisible	664ms	798ms	134ms	0.8	16.8%
buildMDXVisible	652ms	736ms	84ms	0.39	11.4%

Greptile Summary

This pull request replaces the use of Array.prototype.at() with standard array indexing in the Vite builder's code generation script, improving compatibility with older browsers.

• Modified code/builders/builder-vite/src/codegen-modern-iframe-script.ts to use standard array indexing instead of .at()
• Addresses issue [Bug]: Storybook Vite Builder injected runtime uses .at() method, breaking compatibility with older browsers. #29030 by ensuring compatibility with pre-2022 browser versions, particularly iOS Safari <15.4
• Improves Storybook's accessibility on older devices without compromising functionality
• Minimal change with significant impact on browser compatibility for Vite-built Storybook instances

[greptile-apps]

LGTM

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