feat: Add --chromium-pref to CLI options

[PatrykKuniczak]

Basing on #2912, I've taken couple commits and a bunch of code.

I've added request changes of @Rob--W review.

I've converted chromiumPrefs to Map() because of https://github.com/mozilla/web-ext/pull/2912/changes#r1704363575
I hope you'll like that, because you've suggested to convert only coerceCLICustomChromiumPreference func, but i thought it's good idea to use Map() like it's now, but maybe that's not good idea.

I see

web-ext/src/firefox/preferences.js

Line 120 in 308b344

const customPrefs = {};

have object

And there's a question, are you want my solution and probably want to convert coerceCLICustomPreference and entire pref to be Map()(In other PR) or want me to revert that change and use object for all places excluding coerceCLICustomChromiumPreference or maybe you want 1 approach here and other in coerceCLICustomPreference(IMO this should be consistent)

Let's guide me, how to merge this PR successfully, because a bunch of community of wxt-dev/wxt#137 waiting for this, and i want to do it and use in our code 😸

I hope this is understable and nothing'll block me to do this 😆

[PatrykKuniczak]

Wait a second coerceCLICustomChromiumPreference is the same as coerceCLICustomPreference, but there's const customPrefs = {};

I'll remove coerceCLICustomChromiumPreference because that's redundant, but @Rob--W tell me, if you want Map() like it's now, and i'll convert coerceCLICustomPreference to use Map() and remove regexp.

I'm waiting to your response and then i'll quickly adjust this PR 😄

[Rob--W]

This PR makes more changes than strictly necessary to implement the functionality, such as moving several functions around without obvious reason.

Could you undo these unnecessary changes to make it easier to review the code? It has been a long while since I have reviewed the original unmerged PR so I basically have to review from scratch because the PR has escaped my human context window in the meantime.

[PatrykKuniczak]

@Rob--W Which changes are you want me to undo?