This commit adds support for hooks that lets a user integrate another parser with `micromark-rs`, to parse ESM and expressions according to a particular grammar (such as a programming language, typically JavaScript). For an example integrating with SWC, see `tests/test_utils/mod.rs`. The integration occurs with two functions passed in `options`: `mdx_expression_parse` and `mdx_esm_parse`. The can signal back to micromark when they are successful, whether there is an error at the end (in which case micromark will try to parse more), or whether there is a syntax error (in which case micromark will crash).