Parsing arithmetic expansion

[magicant]

• Define syntax
• Define errors for arithmetic expansion parser
• Implement a version of Lexer::text that handles nested parentheses
• Implement Lexer::arithmetic_expansion
• Call Lexer::arithmetic_expansion from Lexer::dollar_word_unit
• syntax.rs: TODO Arithmetic expansion in text_unquote_with_quotes

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https://github.com/magicant/yash/blob/150f39d8dc4dd32af9ce54d53dfdec5df2ae7ea3/tests/arith-y.tst#L5-L8

POSIX says, "The expression shall be treated as if it were in double-quotes, except that a double-quote inside the expression is not treated specially." This means single- and double-quotes are not special and backslashes are special only before a dollar, backquote, backslash, or newline.