Allow spaces between number and unit in formatter

What does this MR do and why?

Allow spaces between number and unit in formatter

This change adds a new configuration option to number formatters to have spaces (or other strings) separating the formatted number from units

The new option is called unitSeparator, and it can be passed to formatters that use units, for example:

// default behavior
expect(kilobytes(1, 0)).toBe('1.0kB');

// new behavior
expect(kilobytes(1, 0, { unitSeparator: ' ' })).toBe('1.0 kB');

Screenshots or screen recordings

NA

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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Related to #337415 (closed)