LOOM-1540: tsx migration for splitinput and inputField components

[Parisistan]

Giving splitInputs some typescript LOVE. Add form test for splitInput

FYI: Had to ignore a lint for one of the test because otherwise we'd need to write a recursive promise chain to do it and it's unnecessarily complicated for this simple test. 🙂

Remember to include the following changes:

• [ ] Ensure the PR title includes the name of the component you are changing so it's clear in the release notes for consumers of the changes in the version e.g [KOA-123][BpkButton] Updating the colour
• [ ] README.md (If you have created a new component)
• [ ] Component README.md
• [ ] Tests
• [ ] Accessibility tests
  • The following checks were performed:
    • [ ] Ability to navigate using a keyboard only
    • [ ] Zoom functionality (Deque University explanation):
      • [ ] The page SHOULD be functional AND readable when only the text is magnified to 200% of its initial size
      • [ ] Pages must reflow as zoom increases up to 400% so that content continues to be presented in only one column i.e. Content MUST NOT require scrolling in two directions (both vertically and horizontally)
    • [ ] Ability to navigate using a screen reader only
• [ ] Storybook examples created/updated
• [ ] For breaking changes or deprecating components/properties, migration guides added to the description of the PR. If the guide has large changes, consider creating a new Markdown page inside the component's docs folder and link it here

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[github-actions[bot]]

	Warnings
:warning:	Package source files (e.g. `packages/package-name/src/Component.js`) were updated, but snapshots weren't. Have you checked that the tests still pass?
:warning:	Package source files (e.g. `packages/package-name/src/Component.tsx`) were updated, but type files weren't. Have you checked that no types have changed?

Browser support

If this is a visual change, make sure you've tested it in multiple browsers.

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[olliecurtis]

With the new change to this property this will change the contract with consumers, so we will need to move the label on this PR to be major :)

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