This PR fixes parsing and error reporting for the regex range quantifiers for expr. The \{ token is handled literally when used at the start of the regex or its subexpressions. The following closing braces \} are also handled literally until the next \{ that is not at the start of an expression.

Quote from #8009:

Normally, \{ begins a range quantifier like {n,m}. However, at the start of the regex or its subexpression (after \( or \|), there is no preceding item to which the quantifier can be applied. The expr command should handle \{ literally at the start of the regex or its subexpression.

Other changes:

• Remove nonexistent error UnmatchedClosingBrace
  • The closing brace without a related opening brace is handled literally
• Fix error message for too big range quantifier index
  • "Invalid content of {}" --> "Regular expression too big"
• Simplify transpiling input regex pattern from BRE syntax to onig crate's Syntax::grep

fixes: #8009