chore: standardise context objects pattern

[kurtdoherty]

This PR

• Adds new guidelines doc for context objects
• Replaces dialogRef for Drawer and Dialog context objects with simple getClosestDialogElement helper function

[rpt-uk-github]

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Coverage variation	Diff coverage
Report missing for 47bd94a1	✅ 100.00%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (47bd94a)	Report Missing	Report Missing	Report Missing
Head commit (81d20ef)	9260	8566	92.51%

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Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#777)	40	40	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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