Replace Ext_json with yojson

[cknitt]

As suggested in #7435 and various other places, we should move from our "homegrown" JSON parser to yojson.

[nojaf]

We should similarly replace some of the analysis lsp types with https://opam.ocaml.org/packages/lsp/.
This also uses yojson under the hood.

[cknitt]

Ext_json is removed here: #8365

But there is also jsonlib (analysis/vendor/json)

[cometkim]

Hmm, #6475 should be abandoned then