[vscode] Add WebviewOptions.enableForms

[sgraband]

What it does

Added the optional enableForms field to WebviewOptions.
This allows the allow-forms sandbox attribute to be set.

Contributed on behalf of STMicroelectronics

How to test

To test you can use the following VSCode extension:

webview-forms-0.0.1.zip

It contributes 6 commands (category: Test enableForms) that will create a webview with different options set (You can see the options set in the command name and the webview will display them as well).
When you opened the webview you can inspect the DevTools to see that the allow-forms sandbox attribute is correctly set (or not set).

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[lucas-koehler]

Code changes LGTM. Suggested tests with the provided example extension worked as expected for the inner iframe. I don't know if the outer one is even expected to have adapted sandbox properties as well. These are set via element.sandbox.add('allow-scripts', 'allow-forms', 'allow-same-origin', 'allow-downloads'); in webview.ts

[sgraband]

Since the outer frame is also not changed by the enableScripts field my understanding is that we need all of these options to even be able to load the main.js script inside. So my proposal would be to just discuss this in the upstream PR if this needs changes. Is this fine with you?

[lucas-koehler]

@sgraband Yes that is fine with me. I agree that most likely no changes are necessary there but wanted to point it out just to be sure there is no oversight :)