Improve snippet sanitization

[Soupstraw]

Currently the constructor of CompletionItem expects a Maybe Text value in the _insertText field, but that leaves the possibility of outputting malformed snippets as is the case in hls. The LSP specification defines the syntax for snippets and I think there should be a datatype to represent this syntax tree. This would be really useful for sanitization, where we could implement a function that converts this data type to a sanitized string.

What I'm suggesting is to add new data types:

newtype Snippet = Snippet [SnippetAny]

data SnippetAny
  = SText Text
  | SPlaceholder Int SnippetAny
  | ...

and then change the type of the _insertText field to Snippet. Then when CompletionItem is serialized to JSON, it applies a function sanitizeSnippet :: Snippet -> Text that converts this Snippet value to a valid LSP snippet. This Snippet type has a rich enough structure to make it easy to implement proper sanitization as outlined in the spec:

With \ (backslash), you can escape $, } and . Within choice elements, the backslash also escapes comma and pipe characters.

[Soupstraw]

Oh, ok looking at the code, it seems that CompletionItem is in a generated source file, so maybe that's not viable. I was just trying to implement a solution to the hls issue I linked to, but I felt that for a proper solution it makes sense to make changes to this library instead.