feat: Add custom callback variant of LspRequestCustom

[blami]

This change allows to provide callback function to custom requests.

Builds on #640 to accommodate scenarios where response is needed from server but cannot be handled by custom handler (as there's no method field).

[blami]

I was wondering if it would be any useful to have also sync and async variant of RequestCustom (e.g. RequestCustomAsync). With these two virtually any missing/niche functionality can be done as custom request.

I was also thinking of providing noop g:LspRequestCustomReply (basically same as WorkspaceExecuteReply except scope) that can be used if caller does not care about response.

Let me know what do you think and I can make those changes.

[jclsn]

Afaik a funcref needs to exist. For every call without a callback, you would have to create and empty function to pass as argument. So I think it would be better to have a separate function for this. Not a 100% sure though.

[blami]

@jclsn Thanks for review, I agree with you. I like approach with *Cb() callback variant. Also I changed the non-callback to just call callback variant providing original WorkspaceExecuteReply so that if logic ever changes fix will be in single place.

The only other option I can think of is to have variadic argument and check for emptyness but that feels too fragile and obscure to me.

Let me know what do you think now.

[jclsn]

Looks good to me, but in the end @yegappan needs to approve.

[yegappan]

Can you update the help text with some details about how to use this new function?

[blami]

@yegappan @jclsn Sure I can. I actually did bit of investigation and learning and feel in my specific usecase this should not be needed and it is language server for which I do this being wrong and anything that edits workspace should be really code action and not custom request.

Anyway I think there's still some use for this and g:LspRequestCustom. As I got more familiar with codebase I now think that rather than making both these g: it would be actually better to export in lsp.vim as in case of other functions that are used by commands.