server: more specific types for decorators

[perrinjerome]

Description

Use a TypeVar so that type checkers understand that the signature is
same as the decorated functions, so that these functions can be
typechecked.

fixes #88

Code review checklist (for code reviewer to complete)

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Commit messages are meaningful (see this for details)
• Tests have been included and/or updated, as appropriate
• Docstrings have been included and/or updated, as appropriate
• Standalone docs have been updated accordingly
• CONTRIBUTORS.md was updated, as appropriate
• Changelog has been updated, as needed (see CHANGELOG.md)

[danixeee]

@perrinjerome And thank you again! :)

After we merge #92, please update the changelog.

[perrinjerome]

I realised that the @thread needed the same change, otherwise we loose the type when used like this:

@json_server.thread()
@json_server.feature(COMPLETION, trigger_characters=[','])
def completions(params: CompletionParams = None) -> CompletionList:
    """Returns completion items."""
    return CompletionList(False, [
        CompletionItem('"'),
        CompletionItem('['),
        CompletionItem(']'),
        CompletionItem('{'),
        CompletionItem('}')
    ])

I pushed a fixup commit for that.

I'll rebase to squash commits and update changelog once #92 is merged (let me know if you don't like rebase and push force, then I can do with merge)

[perrinjerome]

@danixeee I rebased and updated changelog, please take another look

[danixeee]

Looks good.