Fix token value autosize

[macintoshhelper]

Why does this PR exist?

Fixes #2721

What does this pull request do?

If users try to enter something like a long linear-gradient value and the value goes quite wide, the text input currently is not big enough to make sense of anything. Let's find a way where the input grows in height when users enter more.

MentionsInput height CSS is changed to auto; MentionsInput already has the autosize prop passed to it.

We need to watch out for the prepending and appending button to the left so that those are still displayed correctly (likely vertical alignment top)

Handled by adding height: auto CSS to <StyledPrefix ... />. The prefix is automatically vertically aligned to the center.

Testing this change

1. Open Tokens Tab
2. Scroll down to Typography
3. Create a Token with a very long name
4. Click + (Add New Token)
5. Under the Typography fields, try entering very long values – verify that the input textarea expands to multiline
6. Change to Reference Mode
7. Click the dropdown button, or mention the very long Typography token by typing { to bring up the mention dropdown
8. Verify that the input textarea expands to multiline

Additional Notes (if any)

Before	After

[changeset-bot]

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Latest commit: a791dd6

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[six7]

Nice!

[mikekamminga]

Looks good 🚀