Fix: Convert variable-length lookbehinds to PCRE-compatible form

[Chocapikk]

Summary

Fix PCRE-incompatible lookbehind and two broken include references.

Changes

PCRE compatibility

The normal_statement.begin regex wraps lookbehind alternatives in a non-capturing group, which prevents PCRE from auto-splitting:

# Before (PCRE cannot auto-split through the (?:...) wrapper)
(?<=(?:^|;|\||&|!|\(|\{|\`))

# After (PCRE auto-splits top-level alternatives)
(?<=^|;|\||&|!|\(|\{|\`)

Broken includes

Two include references point to non-existent repository keys:

• #arithmetic_dollar -> #arithmetic_no_dollar (fixes arithmetic scoping in interpolation)
• #line_continuation_character -> #line_continuation (fixes backslash continuation in comments)

Why

There is a PR on github-linguist/linguist to replace the archived atom/language-shellscript grammar with better-shell-syntax for GitHub.com syntax highlighting. These fixes are required for that integration.

[RedCMD]

are you sure PCRE doesn't support ^ in a lookbehind?
(?<=^|;|&|\s)
AFAIK PCRE and Onigmo both support top level variable-length-lookbehinds
as they automatically split it into multiple constant-length-lookbehinds

out of all of them
I only see one regex that wasn't supported by PCRE

with the actual change being very small

(the rest of the warnings are just broken includes)

even https://regex101.com/r/6uHp7M/1 agrees
https://regex101.com/r/Rw2eEt/1

[Chocapikk]

You're right. I over-fixed this - PCRE handles top-level variable-length lookbehinds by auto-splitting them.

The only actual issue is in normal_statement.begin, where the lookbehind wraps alternatives in a non-capturing group (?<=(?:^|;|\||&|!|\(|\{|\)), which prevents PCRE from auto-splitting. Removing the (?:...)` wrapper fixes it.

I've force-pushed with just that single change. Thanks for catching this.