Fix toSlatePoint() when text node ends in \n in Firefox

[clauderic]

Description

In Firefox, selecting the entire contents of a document via Ctrl+A causes the editor to crash when the document ends with a \n character.

Refer to the issue below for more specific replication steps.

Issue

Fixes: #4373

Example

Before

Before.-.Firefox.Ctrl+A.newline.bug.mp4

After

After.-.Firefox.Ctrl+A.newline.bug.mp4

Context

In Firefox, range.cloneContents() returns an extra trailing \n, when the document ends with a new-line character. This results in the offset length being off by one, so we need to subtract one to account for this.

Chrome:

Firefox

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: 7a18dc4

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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