Add support for defintion lists

[chrisjsewell]

Basically implement https://github.com/markdown-it/markdown-it-deflist in markdown-it-py then allow the extension to be (optionally?) loaded in

MyST-Parser/myst_parser/sphinx_parser.py

Line 196 in 8dda2f7

to_docutils(

[dhermes]

@chrisjsewell Per our discussion on #163, here is an example of a directive I'm drafting for a Go method:

```{go:method} ApplyMigration
:file: manager.go
:line-number: 137
:receiver: *Manager
:param-name 0: ctx
:param-type 0: context.Context
:param-name 1: migration
:param-type 1: Migration
:return-name 0: err
:return-type 0: error

ApplyMigration creates a transaction that runs the "Up" migration.
```

(I'm open to other variants of the param-* / return-* approach. It just needs to be flexible enough to have named and unnamed values in a signature and the actual type can be quite complex. Also, unlike Python or other languages there is no Go convention (yet) for documenting individual parameters so this is strictly a mapping to name/type expression, in order.)

[pradyunsg]

Is this not resolved? -- https://myst-parser.readthedocs.io/en/latest/using/syntax-optional.html#definition-lists

[chrisjsewell]

yep 👍 I would note though that what @dhermes above is after is a field list markodown-it plugin + render method, see https://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html#field-lists

so I'll close this one, but if you want to open a new issue about that, by all means go ahead (I thought there already was one, but apparently not)