Replace a bunch of type-specific Wasm allocation functions with a generic
byte allocator and reusable opaque out-parameters for pointers. This
makes it a lot more ergonomic (relatively) to use the Wasm interface
and removes a dozen or so exports.
This also updates the `ghostty_type_json` `abi` field with a maximum
alignment value that host sides can use to keep every allocation aligned
properly, easily, without hardcoding numbers.
This adds a test to verify this all works as intended and runs in CI.
Remove the dedicated `zig build lib-vt` step and replace it with a
`-Demit-lib-vt` build option. This fixes two problems:
1. We can default XCFramework, app, etc. steps to false if emit-lib-vt
is true, so that the lib-vt build doesn't pull in unrelated
artifacts. **Most importantly, lib-vt alone can be build without
full Xcode installations.**
2. We can build lib-vt as part of a bundle with other artifacts if we
really want.
