fix(windows): fix MSVC shared library CRT linking in libghostty-vt

[eunos-1128]

Problem

Building libghostty-vt as a shared library on Windows with MSVC fails because linkLibC()
only links msvcrt.lib, while shared DLL builds also require vcruntime.lib and ucrt.lib.
This blocks downstream projects (e.g., herdr) from building against libghostty-vt on Windows.

Solution

Explicitly link the full MSVC CRT chain for shared library builds:

• Link libvcruntime
• Add Windows SDK UCRT library path to linker search directories
• Link libucrt

Changes

Modified src/build/GhosttyLibVt.zig to:

1. Detect shared library builds with MSVC ABI
2. Dynamically resolve Windows SDK path and architecture-specific UCRT directory
3. Link missing CRT libraries

This change aligns with an existing fix in GhosttyLib.zig.

Verification

✅ Build succeeds on Windows with MSVC

Related

Addresses the Windows/MSVC build issue encountered when packaging herdr for conda-forge:
conda-forge/staged-recipes#34029

[ghostty-vouch]

Hi @eunos-1128, thanks for your interest in contributing!

This project requires that pull request authors are vouched, and you are not in the list of vouched users.

This PR will be closed automatically. See https://github.com/ghostty-org/ghostty/blob/main/CONTRIBUTING.md for more details.

[pluiedev]

Vouched manually in #13166