feat(lexer): add string interpolation lexing

[ofabiodev]

Summary

Adds complete lexer support for quoted strings and string interpolation.

Changes

• Add string tokens, escape decoding, literal brace handling, and interpolation lexing.
• Add diagnostics and recovery for malformed strings and interpolations.
• Add coverage for nested expressions, inner strings, offsets, and recovery behavior.

Testing

make test
make build
cd src
go vet ./...