Add support for snippets in the language server.

[JoshuaBatty]

Users should be able to run custom commands for generating and adding various code snippets.

@Braqzen had a good idea on Slack that i'm posting below. Would be great to get any more ideas for other handy snippets if people have them.

From Slack:

A way to auto template doc comments on functions. You could specify a file or a function and it would create a basic template which includes your function args so that there's less to do e.g. it could look something like.

/// Function summary here
///
/// # Arguments
///
/// * `parameter name 1` - 
/// * `parameter name 2` -
///
/// # Returns
///
/// * `SomeStruct` - 
/// * `bool` - 
/// * `bool` -

There wouldn't actually be a summary but there would be some placeholder instead. Also, if it returns anything then it would specify that too in the same order, similar to the params.