Update lexer with grammar changes

- Change/fix the raw string syntax
- Remove newline handline (will require user to escape \r\n themselves)
Rephrase error
Parse optional commas in tables
Don't pass root and import to the visitor, but rather explicitly mark the path as absolute/relative/import

Also, disallow 'import' on the RHS.
Add ConfigString::into_string for efficiency
Escape some special characters specially