fix: param placeholders for types with right brace

[theofabilous]

Completions for functions with snippet placeholders are broken when an argument's type is rendered with a right brace (}). The right brace symbol is rendered unescaped, which causes snippet parsers to interpret it as the end of the placeholder, leaving an extra trailing }.

Example:

fn foo(x: struct { x: i32 }) void {
    _ = x;
}

When expanding the completion item for a function call of `foo` with snippet placeholders,
we get the following:

     selected region
     ______________
    |              |
foo(|x: struct {...|})


The snippet is parsed as follows:

      __ placeholder start
     |
foo(${1:x: struct {...}})
                  ^   ^^
                 /    | \_____ not part of snippet placeholder
            ignored   |
                     placeholder end

This occurs (at least) in the following cases:

1. type is an anonymous container literal (enum { ... })
2. type's name is a raw identifier with a } (@"Evil}Name")
3. type is a generic function with a type argument that is itself problematic (std.ArrayList(struct {}), ...)

Fixed by adding an escape_r_braces option to FormatOptions and escaping '}' symbols (as specified by LSP protocol - Snippet syntax) in Type.rawStringify() when this option is set to true. When rawStringifyParameter calls this function, it sets options.escape_r_braces to true if snippet placeholders are enabled.

In Type.rawStringify, I only specifically handle escape_r_braces in cases where I've encountered this bug. There are a few lines in that function that render strings as is (without potentially escaping r-braces) which I didn't touch because I haven't encountered errors related to those code paths (and in most cases, wasn't sure if those code paths are reachable with snippet placeholders and/or can ever yield } symbols).

I did this so that the changes map nicely onto the added test cases (removing any change causes at least one of the added tests to fail).