feat: Add new TableHeaderCell

[kurtdoherty]

Context

• We have table atoms in @reapit/elements/lab/table, but these don't facilitate the level of nuance required by the DS. For example:
  • They are pinned to semantic table elements like td, tr and so on, but we may need the flexibility of using them in a div-based DOM structure instead.
  • They do not provide any solution for the primary row action (which is meant to appear like the row itself is interactive).
  • They do not provide any solution for rendering row header cells.
  • They require column widths to be defined at the cell-level of the table rather than once at the table-level.
• We want to enhance the capability of our table atoms when implementing "official" core versions of them (i.e. the table atoms that will be available via @reapit/elements/core/table.
• The first chunk of this work focused on atoms involved in the table's body and was completed over a number of PRs: feat: Add new TableRowPrimaryAction #715, feat: Add new TableCellPrimaryData #716, feat: Add new TableCellDoubleLineLayout #717, feat: Add new TableBodyCell #718, feat: Add new TableBodyRow #719, feat: Add new TableBody #720 and feat: Add new TableRowMoreActions #721.
• The next chunk of work will be focused on the atoms involved in the table's head:
  • Part 1: feat: Add new TableCellSortButton #725
  • Part 2: Header cell (this PR)
  • Part 3: Header row
  • Part 4: Table head

This PR

• Adds TableHeaderCell.

[rpt-uk-github]

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Coverage summary from Codacy

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Coverage variation	Diff coverage
Report missing for 9b903791	✅ 100.00%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (9b90379)	Report Missing	Report Missing	Report Missing
Head commit (a7f92d1)	8525	7886	92.50%

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Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#726)	17	17	100.00%

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[kurtdoherty]

note: need these temporarily while the proper header row and table head components are built