LetterSpacingControl: Filter out percentage unit from available units

[Mayank-Tripathi32]

What?

Closes #70478

Remove percentage (%) unit option from letter-spacing typography control dropdown.

Why?

Users were seeing "%" as an available unit option in the letter-spacing typography control, but percentage values are not valid CSS units for the letter-spacing property. When users selected percentage values, their letter-spacing settings would not work, causing confusion and frustration.

The CSS letter-spacing property accepts length values (px, em, rem, etc.) but not percentage values, making the percentage option misleading and non-functional.

How?

Modified the LetterSpacingControl component to filter out the % unit from the available units before passing them to the useCustomUnits hook. This follows the same pattern already used in the Spacer block control, which also filters out percentage units where they're not appropriate.

The change:

• Filters the availableUnits array to exclude % when units are provided from settings
• Maintains the existing fallback behavior when no settings are available
• Adds a descriptive comment explaining why percentage units are disabled

Testing Instructions

1. Open a post or page in the block editor
2. Insert any block that supports typography settings (e.g., Paragraph, Heading, Button)
3. Open the block settings panel
4. Click on the Typography section
5. Locate the "Letter spacing" control
6. Click on the unit dropdown next to the letter spacing input
7. Verify that "%" is no longer listed as an available unit option
8. Confirm that other units (px, em, rem, vh, vw) are still available
9. Test that letter spacing values work correctly with the remaining units

Screenshots or screencast

After: Letter spacing dropdown excludes "%" option, showing only functional units

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Co-authored-by: Mayank-Tripathi32 <mayanktripathi32@git.wordpress.org>
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[t-hamano]

Thanks for the PR.

percentage values are not valid CSS units for the letter-spacing property

This may be a Chrome-specific bug, so we'd be cautious before moving forward with this PR.

See #70478 (comment)

[Mayank-Tripathi32]

Closing as the support for % is getting added slowly