feat: rustemo parser generator

[LeanSerra]

Description
This PR adds rustemo as the parser generator implementing the following simple functionality:

• Simple language definition in grammar.rustemo that allows parsing hello_world.lm example
• Writing the output of the lexer to a file
• Modifies lex.l to return rustemo tokens and implements the custom lexer to use rflex with rustemo
• Writes the output of the parser to a file .parser with the rules it recognized

[MatiBarcia]

LGTM