[Feature] Add XML template autofill

[envoidia]

When you type ///, it should automatically add a template for documentation, like so:

autocomplete_xml_zed.mp4

[GustavEikaas]

I'm totally fresh to zed but I do have a lot of Roslyn and C# experience. Seems like this plugin supports multiple LSP's but for the roslyn-language-server there is a method in the LSP called textDocument/_vs_onAutoInsert.

This file contains the lua implementation of invoking it if that helps https://github.com/GustavEikaas/easy-dotnet.nvim/blob/main/lua/easy-dotnet/roslyn/lsp/auto_insert.lua#L10

The flow is something like this

1. trigger a function when /// is written
2. send the lsp request to roslyn
3. insert the snippet the roslyn lsp responded with