Make odoc references in documentation comments navigable

[N1ark]

Fixes #436.

Implement textDocument/documentLink and documentLink/resolve, which report the links an odoc comment contains, so editors underline them and cmd-click opens them:

type t = Foo

(** Know if a value of type {!t} is {!Foo}.
    See {{:https://ocaml.org} the site}.
    @see <https://ocaml.org/manual> the manual *)
let is_foo Foo = true

In the above example, t, Foo, https://ocaml.org and https://ocaml.org/manual are all clickable! Same if you hover the function and click in the markdown docs. Cross-references are resolved lazily. Cross-module and stdlib references work wherever go-to-definition does, i.e. once the project is built; {!Stdlib.List.map} opens the compiler's list.ml.

This is best reviewed commit by commit; all increments build and all tests pass at each step.

The PR is bigger than I initially planned. If it's easier for you I'm happy to split it into ~3 PRs, following the structure given by the commits here!

Implementation Details

The system for handling odoc references is in Odoc_reference. From there, we call Merlin_analysis.Locate.from_string rather than Query_protocol.Locate, as we need to select exactly in what environment we're looking for the reference. The environment is obtained from Typedtree, which is not ideal, but e.g. Locate also uses it, so I felt like it was probably ok, for such a light usage.

All functions in Doc_to_md have a new ?resolve parameter, to resolve references to hyperlinks. The change is a bit churny, sorry about that. If you have a better idea to avoid it let me know :)

Also had to tweak Uri.to_string, it used to percent-encode sub-delimiters in queries and fragments, so a URL like https://x.dev/s?q=a&n=1 was escaped as https://x.dev/s?q%3Da%26n%3D1, which is wrong.

Note

I acknowledge this PR was mostly written by an LLM. I took care of directing it, and reviewing the changes along with cleaning up parts of the code and comment.