[IndexTable] Sticky header in first or last fixed column missing border and getting incorrect min-width

[chloerice]

Issue summary

The header of IndexTable is missing fixed column header border when scrolling, they're the wrong width, and the last column's header isn't sticky.

Expected behavior

First sticky header cell should:

• Have correct width (needs to have padding-right 400 and min-width set on style prop should round up to the 1px)
• Have border-right when scrolling

Last sticky header cell (when last column sticky) should:

• Be sticky
• Have correct width (needs to have padding-left 400 and min-width set on style prop should round up to the nearest 0.5px)
• Have border-left when scrolling

Actual behavior

First sticky header cell:

• Has incorrect width (has padding-right 300 and min-width set on style prop is 1px lower than column's width)
• Doesn't have border-right when scrolling

Last sticky header cell (when last column sticky):

• Isn't sticky (scrolls with the other header cells)
• Has incorrect width (has padding-right 300 and min-width set on style prop is 0.5px lower than the column's width)
• Doesn't have border-left when scrolling

Steps to reproduce the problem

Go to a store with an order list long enough to be scrolled so the IndexTable header is sticky, note the first column header cell is missing a left border and is not the correct width on inspection compared to the column data cells.

[jesstelford]

Looks like the sticky header has significantly diverged from the non-sticky header, and so it's missing logic and styles that the non-sticky header has since gained.

In addition to the OP screenshots, we noticed the sticky header's first column is always staying sticky when the non-sticky header doesn't due to insufficient horizontal space:

After investigating with @gwyneplaine, it appears the best path forward is to re-use the logic & styles from non-sticky header in the sticky header. This should bring back consistency to fix the discovered issues and ensure it doesn't drift again.

Brain dumping some tasks:

### Tasks
- [x] ~Remove unused `.Table-sortable` background styles (Summer Editions made that redundant)~ Moved to: https://github.com/Shopify/polaris-internal/issues/1465
- [x] Add a `Tag` option to `renderHeading()` to toggle between `div` / `hr`
- [x] Remove `.StickyTableColumnHeader` styles
- [x] ~Consolidate `.StickyTableHeader` styles with `.Table` styles~ _(out of scope)_
- [x] Consolidate DOM element width calculations / `min-width` setting inside `resizeTableHeadings()`
- [x] Search for selectors targeting removed classes
- [x] Test with Bulk actions
- [x] Test with loading state
- [x] Test when there is/isn't enough width to render the sticky columns (Add story variants for this? Maybe by rendering in a fixed-size scrollable container?)
- [x] Test when `unselectable === true`
- [x] Test / compare accessibility
- [x] Test sortability