[6.x] Improve Combobox performance

[duncanmcclean]

This pull request aims to improve the performance of the Combobox component, specifically when opening/closing the dropdown and searching through lots of options.

Improvements:

• Refactored isSelected method to use .some() instead of .filter() to avoid unnecessary comparisons.
• Removed the deep cloning of options in filteredOptions
  • I can't seem to figure out why I added this in the first place, but everything seems to work fine without it.
• The measureOptionWidths method now only measures the width of the three options with the longest labels, not every option.
• Enabled the force-mount prop for longer dropdowns, allowing options to be rendered & measurements to happen before the user clicks to open the dropdown.
  • This change seems to have the biggest impact of them all. The Icon fieldtype opens almost instantly, compared to waiting 400(ish) ms before this PR.
• Fixed an issue where the dropdown's min-height was dependant on the options prop, not filteredOptions.
  • This meant that if you had 10 options, but filtered down via search to just 1, the min-height for 3 and above would remain applied.

Closes #13615