[Feature request] Capture groups in syntax definition files

Some languages require knowing a previous token to be able to correctly highlight them. For example:

• Lua allows putting an arbitrary amount of equals signs = between the two square brackets for long strings ([[ ... ]]) and block comments (--[[ ... ]]). The equals signs must be balanced (same amount for the opening and closing token).
• C# allows using more than tree double-quote characters """ for long strings literals.
• Unix shells (Bash, Zsh, etc.) allow creating "heredoc" strings with the << operator:

  cat << EOF
  This will be treated as text, piped into cat's standard input.
  Variables like $x may be expanded here.
  EOF
  

In the case of shell heredocs, the problem is worse because the text after << can be anything.

Thus, my suggestion: Allow YAML grammar files to use captures with $1, $2 and so on, similarly to how it currently works with the replaceall command.

Start by tracing YAML grammar handling and the existing replaceall command, since the issue identifies those as the relevant behavior. Define how numbered captures should work, then verify the Lua, C# and shell heredoc examples with tests covering balanced delimiters and arbitrary heredoc markers.