Refactor GhosttyLibVt to support both shared and static library
builds via a shared initLib helper that accepts a LinkMode. The
shared and static entry points (initShared, initStatic) delegate
to this common path.
For static builds, compiler_rt and ubsan_rt are bundled to avoid
undefined symbol errors. Debug symbols (dsymutil) are skipped for
static libs since they are not linked. The install artifact uses
a "-static" suffix internally but installs as "libghostty-vt.a"
via a new installLib method. Wasm is excluded from static builds
since it has no meaningful static vs shared distinction.
