docs: document PerBrowser options for entrypoints (#2304)

[dashitongzhi]

Closes #2304.

Adds a 'Per-Browser Options' section to docs/guide/essentials/target-different-browsers.md
explaining PerBrowserOption<T> / PerBrowserMap<T> usage on entrypoint
config fields like matches, runAt, and defaultArea.

• Explains the type shape: T | { [browser: TargetBrowser]: T }
• Lists the entrypoint fields that accept it (matches, runAt, matchAboutBlank, excludeMatches, includeGlobs, excludeGlobs, allFrames, matchOriginAsFallback, cssInjectionMode, registration, defaultTitle, defaultArea, persistent)
• Shows both forms (single value, per-browser map) with concrete defineContentScript examples
• Notes the fallback behavior when a build target is omitted from the map
• Adds a tip about mixing both forms inside the same entrypoint
• Cross-references Manifest config for fields that don't fit the per-key shape (e.g. default_icon)

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[Copilot]

Pull request overview

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Adds documentation for configuring entrypoint options per build target (browser), explaining the PerBrowserOption/map pattern and providing examples.

Changes:

• Introduces a new “Per-Browser Options” section describing per-browser option values and maps
• Adds examples for using a single value vs. per-browser maps
• Notes fallback behavior when a target is omitted and links to a manifest-level workaround

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.51%. Comparing base (7905cb3) to head (233e1f1).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2403      +/-   ##
==========================================
+ Coverage   78.46%   78.51%   +0.05%     
==========================================
  Files         133      133              
  Lines        3882     3882              
  Branches      875      875              
==========================================
+ Hits         3046     3048       +2     
+ Misses        749      747       -2     
  Partials       87       87              

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commit: 233e1f1

[PatrykKuniczak]

@dashitongzhi I think it's alright, but wait for Aaron approval.

But i think we should make TargetBrowser not string but put a literal with browsers name there:

wxt/packages/wxt/src/types.ts

Line 569 in 7905cb3

export type TargetBrowser = string;

Feel free to open another PR.