Allow plugins to modify server response messages

[jwortmann]

This method for AbstractPlugin allows LSP-* plugins to modify the "result" in (successful) response messages from the server, before it gets further processed by the response handler function in LSP.

It is useful especially for problematic content in hover responses, but the implementation is generic enough to allow plugins to do any shenanigans they like with the payload ;)

Examples:

Hover popups from LSP-pyright have some minor issues (sublimelsp/LSP-pyright#42), for example there is occasionally only a single \n between the end of a fenced code block and --- (horizontal rule). This is absolutely fine in CommonMark, but mdpopups (or the Python-Markdown parser or whatever it uses) mistakenly interprets it as a Setext header, i.e. currently the code will be bold and the horizontal rule is omitted:

Add the following lines to LSP-pyright in plugin.py, and it will be displayed correctly:

    def on_server_response(self, method: str, response: Response) -> None:
        if method == "textDocument/hover" and isinstance(response.result, dict) and response.result.get("contents", {}).get("kind") == "markdown":
            response.result["contents"]["value"] = re.sub("\n---", "\n\n---", response.result["contents"]["value"])

---

I would like to use it for the hover popups from the Julia language server, which have several similar issues in their Markdown content. In particular they often use links formatted in the form [`title`](@ref), and as soon as you click on the link, it will start a webbroser and try to open @ref as URL, which of course doesn't work. But with small modifications I could make these links to open a corresponding documentation page from the server in a HtmlSheet - https://github.com/sublimelsp/LSP-julia/blob/50f978826407faf0d1aaebcd3c2f8e5cb53f8136/plugin.py#L543-L562