Store the parsed tree to avoid unnecessary re-parsing

[vinistock]

Reflecting on @paracycle's comment, I think delivering this refactor partially will make the transition easier in the future.

This PR changes our storage to store a new helper class Item (let me know if you have better naming suggestions), which keeps the parsed tree and source. For the time being, it must also keep the parser instance so that we can figure out absolute column positions. Once the limitation is addressed in syntax_tree, we can drop the parser and keep only tree and source.

This will probably make it easier to refactor requests later when we can drop the parser. Additionally, it makes it easier to handle syntax errors in a single place in the code (currently the LSP breaks every time you start typing something).

After this, we will only re-parse files when we receive didOpen and didChange requests. For every other request, we just re-use the existing tree.