feat: Use i18nStrings as fallback flash icon labels

[gethinwebster]

Description

Fall back to component-level i18nStrings if individual flash item labels aren't provided

Related links, issue #, if available: n/a

How has this been tested?

New unit tests

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (fc92170) 93.56% compared to head (87ec2f1) 93.56%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1426   +/-   ##
=======================================
  Coverage   93.56%   93.56%           
=======================================
  Files         631      631           
  Lines       16880    16888    +8     
  Branches     5570     5577    +7     
=======================================
+ Hits        15793    15801    +8     
  Misses       1014     1014           
  Partials       73       73           

Files Changed	Coverage Δ
src/flashbar/collapsible-flashbar.tsx	98.02% <ø> (ø)
src/flashbar/flash.tsx	100.00% <100.00%> (ø)
src/flashbar/non-collapsible-flashbar.tsx	100.00% <100.00%> (ø)

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