add completion item snippet mode

[brettfo]

For C# add both angle brackets (<>) for generics and parentheses (()) for methods.

For F# only add parentheses (()), but not generic angle brackets because the F# way is to prefer type inference.

The end result when accepting completions in a notebook is to effectively add tab stops ($1, etc.), so e.g.,

Method completion in C#:

1. The user types: Console.WritL.
2. From the completion list they see and select WriteLine and commit it with <TAB>.
3. The editor now looks like: Console.WriteLine(|) (where the pipe is the cursor). The user can type inside the parentheses and when they're done they hit <TAB> again and the cursor is now at the end of the line.

Generic type completion in C#:

1. The user types: System.Collections.Generic.IEnumerab.
2. From the completion list they see and select IEnumerable<> and commit it with <TAB>.
3. The editor now looks like: System.Collections.Generic.IEnumerable<|> (where the pipe is the cursor). The user can type inside the angle brackets and when they're done they hit <TAB> again and the cursor is now at the end of the line.

Generic method completion in C#:

1. The user types: System.Array.Empt.
2. From the completion list they see and select Empty<> and commit it with <TAB>.
3. The editor now looks like: System.Array.Empty<|>() (where the pipe is the cursor). The user can type inside the angle brackets and when they're done they hit <TAB>.
4. The editor now looks like: System.Array.Empty<int>(|) (where the pipe is the cursor). The user can type inside the parentheses (or not) and when they're done they hit <TAB> again and the cursor is now at the end of the line.

Method completion in F#

Exactly the same as the C# scenario above; parentheses are added for a call to something like System.Console.WriteLine().

Module function completion in F#

Functions defined on a module are commonly curried and parentheses are not preferred. E.g.,

1. The user types: [1;2;3] |> List.ma.
2. From the completion list they see and select map and commit it with <TAB>.
3. The editor now looks like [1;2;3] |> List.map| (where the pipe is the cursor). No parentheses were appended because a function reference is often directly added without parentheses. E.g., [1;2;3] |> List.map someFunction.

Protocol changes

Borrowing the type shape from LSP, only one property, insertTextFormat, was added to the CompletionItem type and it's optional so an older server giving completions to the front end will be unchanged.