[Feature Request] Back reference can be used in nested expressions.

[soasme]

Is your feature request related to a problem? Please describe. Consider an HCL2 heredoc template,

a = <<EOT
something
EOT

We will need to determine if inner lines of Heredoc template is not started with EOT, hence

HeredocTemplate = "<<" Heredoc (!(\1 Newline) .)* \1 Newline;

As of v1.16.0, there is no way to get a back reference inside (...)* as it's under its own Sequence scope. See https://github.com/soasme/PeppaPEG/blob/main/configs/hcl2.peg.

Describe the solution you'd like

• The number of back reference "\N" always refers to the Nth member in the peek sequence rule.
• If inner backref is needed, we can extend the syntax to \M.N, meaning Nth member in Mth member.
• The number only applies to a sequence.

Describe alternatives you've considered N/A

Additional context N/A

[soasme]

\M.N is unnecessary as users can always use \N in a named rule sequence.