Clean up parser diagnostics and token positions

[grian32]

Problem

Several parser/lexer diagnostic details are misleading. currError reports the peek token instead of current token, double-character tokens initialize EndCol with a line number, and struct init errors mention ].

Tasks

• Fix currError to report currToken.Type.
• Fix double-char token position initialization.
• Fix struct init parse error text.
• Add tests that assert diagnostic messages/positions for representative parse errors.

Refs: parser/helpers.go:57, lexer/lexer.go:270, parser/expr_infix.go:124.