feat: Add firefox and chrome variable in @wxt-dev/browser

[SimYunSup]

Overview

Add chrome and firefox variable for specific logic. I'm not sure if this implementation is correct, so I'm going to submit it as a Draft PR.

Manual Testing

Related Issue

Related #1652

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[SimYunSup]

@aklinker1 I wanted to find type-safe import.meta.env type guard, but there wasn't.
So I added just Firefox type. Is there any that I have to implement based on issue?

[aklinker1]

I'm not sure if I like this change.

• What about safari?
• Breaking the standards - projects migrating to WXT may not know which variable to use, and projects migrating away from WXT now have more work to do.
  • Counterpoints:
    • They could just install the @wxt-dev/browser package and continue to use it
    • WXT already uses browser, which most projects don't use.
• I would rather have a single variable include all the types, rather than exporting multiple variables each with their own types.
• Why add a variable for chrome when the browser variable is already typed using @types/chrome?
• What happens to this logic when Chrome releases the browser global for compatibility? if (firefox) ... else if (chrome) ... would not work anymore since it would always be firefox.

Let's continue the discussion on #1652 before doing any more work here. I'm not convinced this is the correct solution yet.