build: add ghostty-internal pkg-config modules (shared + static)

[deblasis]

Summary

Mirror the libghostty-vt-static pkg-config pattern from #12210 for the internal library.

• Add ghostty-internal.pc (shared, -lghostty) and ghostty-internal-static.pc (static, direct archive reference) so consumers can discover either variant via pkg-config
• Named ghostty-internal to distinguish from the public libghostty-vt API
• Static module points at the platform-correct archive name (ghostty-static.lib on Windows, libghostty.a elsewhere)
• pkg-config files are generated during shared builds and installed via GhosttyLib.install()

Test plan

• zig build succeeds (default target)
• ghostty-internal.pc and ghostty-internal-static.pc appear in zig-out/share/pkgconfig/
• Static .pc points at ghostty-static.lib (Windows) / libghostty.a (Unix)
• Shared .pc uses standard -L -l flags
• Existing libghostty-vt pkg-config files are unaffected

[deblasis]

If this one lands, next one would be to create a "fat archive" for windows as well.
Like libtool does on darwin.

I will 90% hit a wall with NativeAOT on windows with winui3 but regardless, that's something MS is supposedly working on.
I might chime in to give a hand with repro/bugs etc.

Update

I hit the wall eventually but found workarounds and managed to get a single executable for Ghostty Windows!
32MB single exe with libghostty (and the other deps) baked in. No ghostty.dll needed.

I filed an issue upstream for anyone else bumping into the same problem: microsoft/WindowsAppSDK#6394

[mitchellh]

Lets rename these to ghostty-internal and ghostty-internal-static because this version of "libghostty" is

1. Not meant for external use
2. Not libghostty and confuses people for it.

[mitchellh]

Blocked on some naming.

[mitchellh]

Let's rename this too. And anywhere else we write it out, let's use this PR to fix it up.