Improve indexing behaviour (for editors other than VS Code, e.g. Zed)

[andyw8]

:wave: As previously discussed by @vinistock in this Zed issue:

We currently don't update the index on textDocument/didChange, which means new declarations are only available once you save the file (after we receive the didChangeWatchedFiles notification). We do need to change that and start indexing on didChange too, so that unsaved declarations show up.

It's also been noted that for Zed at least, saving the file doesn't re-index it, you need to reload the editor.

I may open a PR for this, just wanted to log it first for easier reference and discussion.