Add support for folding ranges

[rtetley]

Fix the folding ranges by letting the server send the ranges contained in Proof. [...] Qed. Or Proof. [...] Admit..
See: https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocument_foldingRange

[rtetley]

Related to #797, it will use the same mechanism

[Durbatuluk1701]

Can we include a concrete example on what this folding range should look like? I personally feel like the current (2.2.3) folding for Proof bodies is good but maybe am missing a corner case

[rtetley]

I believe currently this is only handled by vscode through indentation. If you don't add indentation to your proof blocks it won't work (see #137). Also this could add support for folding modules and sections.