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.
