Syntax Highlighting: Support Multiline Strings

[marfranke]

Taken from @usebruno/lang:

   // Multiline text block surrounded by '''
  multilinetextblockdelimiter = "'''"
  multilinetextblock = multilinetextblockdelimiter (~multilinetextblockdelimiter any)* multilinetextblockdelimiter

  // Dictionary Blocks
  ...
  value = multilinetextblock | valuechar*

Still not supported by vscode syntax highlighter.

[Skeeve]

Should be solved by PR #26

If you want to patch your installation to have the syntax highlighting already available, use the file shown here in my repo to replace your local syntaxes/bruno.tmLanguage.json.