Update C# onEnterRules to account for documentation comments

[dibarbet]

Resolves dotnet/vscode-csharp#8025

A couple months ago it looks like, an onEnter rule was added to 'continue' a comment if enter is pressed inside a line comment. This is generally fine, however it causes problems when writing documentation comments which use /// instead. You end up with something like the following (see also video in linked issue):

/// <summary>
/// This is a documentation 
// comment
/// </summary>

This PR modifies the line comment enter rule to avoid inserting the line comment if it detects a triple comment.

Additionally, I added an on enter rule for /// to auto insert /// when hitting enter anywhere inside a documentation comment. Currently that is handled in the server side of the C# extension, but it can be slow and finicky. Better to just have the client handle it via an on enter rule.

This shows the new behavior:

[dibarbet]

@microsoft-github-policy-service rerun

[shal44m]

@microsoft-github-policy-service rerun