Allow users to use custom colors

[66BA9Q-ME49]

Description

I saw in this comment on #1025 that you want to limit the available colors for tabs and panels to those of Firefox containers.

Could you at least add a way for users to select custom colors, either using a color picker or just by allowing users to enter a hex color code in a field?
I've looked at the code and it should be very easy to implement this feature.
In tab.vue#L104, if the value of tab.customColor is not a valid browser.ColorName, check if it is a valid HEX color and return that hex color if the check passes. Something like this:

if (tab.reactive.customColor) return RGB_COLORS[tab.customColor as browser.ColorName] || (/^#[0-9A-F]{6}$/i.test(tab.customColor) ? tab.customColor : undefined)

Then you just need to add an option in all the custom color dialogs (context menu, "configure panel in sidebar", "configure panel", ...) allowing the user to input a hex code.

Example

I added the following in the devtools:

<div class="opt" data-selected="false" data-separator="false" title="Custom">
  <div class="icon-box">
    <svg class="icon">
      <use xlink:href="#circle"></use>
    </svg>
  </div>
  <div class="TextInput input" data-active="false" data-empty="true" data-valid="false" data-wrong="false" style="overflow: hidden">
    <div class="focus-fx"></div>
    <input autocomplete="off" autocorrect="off" autocapitalize="none" spellcheck="false" type="text" tabindex="-1" value="" />
    <div class="placeholder">HEX Color</div>
  </div>
</div>