Enable fallback to pyls for goto definition of normal python functions

[masahi]

The current setting overwrites the language server with the custom one, so goto definition to normal python functions no longer work. This PR enables fallback to pyls when tvm-ffi-navigator couldn't find the definition.

Also added instructions for eglot.

[tqchen]

Thanks @masahi ! I am not sure it it is the right approach though. I believe that the fallback should be handled by the client side(vscode or lsp-mode).

For example, when a getDefintion request comes into the client, the client could query all the relevant lsp servers and combine their results. In this way there will be more composability, i.e. we can use ffi-navigator with any other python language servers

I believe this is the way vscode handles the multiple lsp server cases, so the original goto def in python also works fine in vscode. Would be nice if we can dig deeper to see if there is such option in lsp-mode, or request/contribute such features

[masahi]

yes, lsp-mode does seem to have multi servers support per lang, but I couldn't quickly figure out how to configure correctly and I also wanted to support other lsp client for emacs, eglot, which takes a minimalist approach and doesn't seem to support multi servers at the moment (I have better experience with eglot for python-mode). This PR is the quickest and most obvious way to fix my immediate need and I am quite happy with my new workflow :)

Feel free to reopen if other people want this.

[tqchen]

Perhaps we should raise the question in the elgot dev to see if it can be easily supported :)?

[masahi]

There is already discussion at joaotavora/eglot#90
The discussion seems to have stopped one year ago though.

[tqchen]

I confirm that multiple server settings works perfectly for lsp