message: Improve end pattern finding
  $$
  x
  $$$y$
  is now handled properly.
diff: @@ -175,11 +175,9 @@ export class CodeActions {
  const text = document.getText(endRange)
  // Check if the end position really contains the end delimiter.
  // This is not the case when the opening and closing delimiters are on different lines
  - const regex = new RegExp('^' + oldDelim.replace(/\$/g, '\\$') + '$')
  - const regexResult = regex.exec(text)
  - if (!regexResult) {
  + if (text !== oldDelim) {
  if (oldDelim === '$$') {
  - const pat = new RegExp(oldDelim.replace(/\$/g, '\\$') + '(?!\\$)')
  + const pat = /(?<!\\)\$\$/
  const endPos = this.extension.mathPreview.texMathEnvFinder.findEndPair(document, pat, endRange.start)
  if (!endPos) {
  return
  