Escape dollar signs in completion snippets

[Soupstraw]

This PR adds sanitization to completion snippets. It does that by introducing the Snippet type and a snippetToText function that converts a Snippet to a syntactically valid snippet string.

close #4363

[sgillespie]

It looks like there are some tests to update:

ghcide
  completion
    topLevel
      recordsConstructor: FAIL (1.06s)
        ghcide-test/exe/CompletionTests.hs:112:
        expected: [("XyRecord",Just CompletionItemKind_Constructor,Just "XyRecord",Nothing),("XyRecord",Just CompletionItemKind_Snippet,Just "XyRecord {x=${1:_x}, y=${2:_y}}",Nothing)]
         but got: [("XyRecord",Just CompletionItemKind_Constructor,Just "XyRecord",Nothing),("XyRecord",Just CompletionItemKind_Snippet,Just "XyRecord {x=\\${1:_x}, y=\\${2:_y}}",Nothing)]

[Soupstraw]

Oh well, I think I'm gonna have to do this properly after all 😃

Completely forgot about record snippets..

[Soupstraw]

Should be good now, I think the test failures are from flaky tests @sgillespie.