Function is_complete 

pub fn is_complete(input: &str) -> bool

Returns true if input is a syntactically complete expression or statement.

Lexes input and counts bracket/brace/paren depth. The input is considered incomplete (returns false) when:

• any parentheses or brackets are unclosed
• braces are unclosed and the input does not end with { (a block opener)

On lex error, returns true so the evaluator can surface the error to the user.