Fix Firefox IME bug caused by placeholder

[12joan]

Description
When composition starts and the placeholder is showing, Slate hides the placeholder. Due to a bug in Firefox, this causes macOS's IME to receive outdated information about the start offset of the composition. This PR works around this issue by rendering the placeholder after the cursor instead of before.

Issue
Fixes #5766

Example

image.mp4

Checks

• The new code matches the existing patterns and styles.
• The tests pass with yarn test.
• The linter passes with yarn lint. (Fix errors with yarn fix.)
• The relevant examples still work. (Run examples with yarn start.)
• You've added a changeset if changing functionality. (Add one with yarn changeset add.)

[changeset-bot]

🦋 Changeset detected

Latest commit: 8d00b2b

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
slate-react	Patch

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[12joan]

The Firefox bug that made this workaround necessary will be resolved in Firefox 141. Noting this here so that anyone reading this in the future knows that this change is now safe to revert if it causes any unwanted side-effects.