Improved detail on invalid string literals

During lexing of invalid string literals, two new error classes have been added:

• InvalidTokenEscapedNullCharInString
• InvalidTokenEoiInString

Lexer provides details about the last error found

Last error details added to lexer, accessible via InvalidTokenReason() method.

Lexer is feature complete

The generated lexer now successfully scans the Cool programming language according to the official spec. All
approval tests are passing against a provided set of official golden files. Additional tests on selected snippets are included for improved debuggability of future PRs.