This change allows building the `cmark-gfm` target for both `wasm32-unknown-wasi` and `wasm32-unknown-wasip1-threads`. The other targets are not tested.

The differences between two target triples:

- `wasm32-unknown-wasi`
  - C
    - can import `unistd.h`
    - has `__wasi__`
    - does not have `__unix__`
    - **does not have `_REENTRANT`**
    - **does not have `_POSIX_THREADS`**
    - **does not support pthreads API at all**
  - Swift
    - can use `#if os(WASI)`
  - SwiftPM
    - `BuildSettingsCondition`'s `Platform` is `.wasi`
- `wasm32-unknown-wasip1-threads`
  - C
    - can import `unistd.h`
    - has `__wasi__`
    - does not have `__unix__`
    - **has `_REENTRANT` (defined in wasi-libc)**
    - **has `_POSIX_THREADS` (defined in wasi-libc)**
    - **supports a subset of pthreads API**
  - Swift
    - can use `#if os(WASI)`
  - SwiftPM
    - `BuildSettingsCondition`'s `Platform` is `.wasi`

This reverts commit 6f64876.

This reverts commit aef4761.

feat: add partial support for WASI

Move `extensions/include/cmark-gfm-core-extensions.h` cpp includes out of extern "C" block

Signed-off-by: Melissa Kilby <[email protected]>

chore: remove unusued GitHub workflows