fix(lexer): implement ANSI-C string quoting support ($'...')

[ravvenox]

The root cause of this syntax-trapping behavior is located entirely in src/grammar/lexer.rs due to a fundamental blind spot regarding Bash’s ANSI-C Quoting ($'...') syntax.

Currently, the lexer handles a standalone $ token by setting self.after_dollar = true and emitting a TokenKind::Dollar. However, because the lexer doesn't recognize the $' sequence as an atomic ANSI-C initialization block, the state tracking falls apart across token boundaries:

1. The Issue with after_dollar Persistence: The lexer consumes after_dollar inside read_var_name() or drops it entirely before realizing that the next sequence is an opening single quote. By the time it processes the quote character ', it treats it as a standard POSIX single quote literal.
2. The Escape Sequence Blind Spot: Inside read_quoted_content(), backslash character processing is heavily guarded behind an is_double_quote check. Because the lexer assumes it is parsing a standard single-quoted string, it completely strips the backslash's escaping power. In a string like $'aa\'bb', the quote following the backslash is mistakenly evaluated as the structural termination quote. The remaining text bb' is left unquoted, and the final quote at the very end opens a brand new, unclosed string context.

This causes DParser::walk to mark the final token as OpeningState::Unmatched, which trips needs_more_input() into incorrectly returning true and forcing a line continuation prompt (> ).

Solution Applied

Introduced a persistent state flag is_ansi_c_quote to the lexer structure. This flag is:

1. Activated at quote-opening time if self.after_dollar is present.
2. Utilized inside read_quoted_content() to conditionally enable backslash escape evaluation for \' and \\.
3. Reset at quote-closing time when the string naturally terminates.

This ensures that echo $'aa\'bb' bypasses the multi-line continuation prompt entirely and executes seamlessly, while preserving pristine POSIX behavior for standalone single quotes where backslashes must remain literal text.

[HalFrgrd]

Thanks for the fix! Some PR feedback: ravvenox#1