feat: Allow positional placement of endSlot in TextField

[scottlovegrove]

Short description

This change allows us to use the bordered variant of TextField and have the endSlot appear centrally, rather than inline with the input at the bottom.

Before:

After (with endSlotPosition set to fullHeight:

I know Design would like this to be the default behaviour, but given I only need it for one area in the app (new onboarding), I was reluctant to make my changes the default behaviour, so I haven't. This is opt-in, rather than opt-out.

PR Checklist

• Added tests for bugs / new features
• Updated docs (storybooks, readme)
• Reviewed and approved Chromatic visual regression tests in CI

[engfragui]

variant: bordered + some endSlot passed in -> I would expect the endSlot to show up regardless of the value of endSlotPosition (even if it's undefined), right? 🤔 (I mean, it should just take the default value for endSlotPosition)

[engfragui]

@scottlovegrove I feel like there are quite a few edge cases here and it's a bit tricky to find them via Storybook. Could we add a couple of unit tests? (Simple ones like the ones inside describe('character count') that we have in text-field.test.tsx?)

[engfragui]

variant: bordered + some endSlot passed in -> I would expect the endSlot to show up regardless of the value of endSlotPosition (even if it's undefined), right? 🤔 (I mean, it should just take the default value for endSlotPosition)

Storybook issue fixed in #902