feat: Add new Dialog.Header

[kurtdoherty]

Context

• The Dialog component has received Design updates; we need to update our implementation to match.
• The work will be done over a series of PRs:
  • feat: Add new Dialog.Footer #628
  • feat: Add new Dialog.Body #630
  • Part 3: Add new Dialog.Header (this PR)
  • Part 4: Update Dialog to use new subcomponents.

This PR

• Adds a new Dialog.Header subcomponent. It is not used or exposed yet, though docs are included for it.

[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 d3a63f21	✅ 96.49%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (d3a63f2)	Report Missing	Report Missing	Report Missing
Head commit (afb7065)	7752	7026	90.63%

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 (#631)	57	55	96.49%

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

1. Codacy didn't receive coverage data for the commit, or there was an error processing the received data. Check your integration for errors and validate that your coverage setup is correct. ↩