[LSP] Do not send 'didSave' notification if language server does not support it

[bluz71]

Spawning from the #3069 PR.

ALE when dealing with language servers appears to be sending the textDocument/didSave notification.

The Dart Analysis Server complains about that in the command line area after every write operation in Vim:

Basically the Dart Analysis Server doesn't support that notification, hence it should not be sent. I think the server on initial handshake indicates what it does support and the client needs to take that into account.

That is clarified by Microsoft here in the LSP spec:

If present will save notifications are sent to the server. If omitted the notification should not be sent

Anyway, this warning is highly distracting and it should not be triggered.

This issue has been noted by other projects and been dealt with:

• Sublime: Don't send textDocument/didSave unless server advertises support for it

• LSC: Don't send textDocument/didSave unless server advertises support for it

• LSP: Unknown method textDocument/didSave

I would be nice if ALE can likewise fix this issue.

Thanks.

[hsanson]

Documentation: https://microsoft.github.io/language-server-protocol/specification#textDocument_didSave

Related: #3925

[hsanson]

@bluz71 appreciated if you can test this PR #3930 and see if it solves this issue.

[bluz71]

@hsanson,

I switched over to native Neovim LSP early this year; so I am not in a position to test this out, sorry.

[akiross]

I have the same issue, so I proceeded with testing #3930 and it seems to solve the issue. Thanks for the PR.

[hsanson]

@akiross thanks for the testing. I have merged the fix now.