caddyfile: Implement variadics for import args placeholders (#5249)

* implement variadic placeholders
imported snippets reflect actual lines in file

* add import directive line number for imported snippets
add tests for parsing

* add realfile field to help debug import cycle detection.

* use file field to reflect import chain

* Switch syntax, deprecate old syntax, refactoring

- Moved the import args handling to a separate file
- Using {args[0:1]} syntax now
- Deprecate {args.*} syntax
- Use a replacer map for better control over the parsing
- Add plenty of warnings when invalid placeholders are detected
- Renaming variables, cleanup comments for readability
- More tests to cover edgecases I could think of
- Minor cleanup to snippet tracking in tokens, drop a redundant boolean field in tokens

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Co-authored-by: Francis Lavoie <lavofr@gmail.com>

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