feat: Add CSS anchor positioning polyfill

[kurtdoherty]

Context

• We're about to refactor our Menu and Tooltip components, with a focus on enhancing their positioning capability.
• CSS Anchor Positioning is not yet supported by all the major browsers (at time of writing, Safari 26 has preview support and Firefox has no support).
• Without CSS anchor positioning, we would have to buy into a significant amount of complexity to implement an equally capable JavaScript positioning solution.
• OddBird provide a CSS Anchor Positioning polyfill that allows CSS anchor positioning to be used on browsers that don't yet natively support it.

This PR

• Implements a new helper, applyCSSAnchorPositioningPolyfill, that allows anchor positioning CSS to be polyfilled on browsers that don't support it.

note: this is currently intended for internal use only, so it not exported in any way from the library for consumers to leverage directly.

[rpt-uk-github]

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[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
Report missing for 012dc821	✅ 100.00%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (012dc82)	Report Missing	Report Missing	Report Missing
Head commit (e0a1284)	7770	7047	90.69%

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#635)	18	18	100.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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Footnotes

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