Add new LCGS parser with better parse errors

[NicEastvillage]

This PR adds a new LCGS parser. Instead of using pom it is now a handmade lexer and recursive-descent parser. This PR is the second step towards #42, #104, and #190.

This PR includes:

• AST nodes for LCGS programs
• Adds parsing of LCGS to new AST nodes
• Testing of new LCGS parser

This PR does NOT replace/remove the old LCGS parser. The old LCGS parser that uses pom is still in use for now as the semantic analysis of IntermediateLCGS (the symbol checking and relabeling) still uses the old AST. This will change in the next PR on this topic.

[falkecarlsen]

Nice!

When migrating to new parser we should be aware if we are backwards-compat. If we are, then only minor-version needs to be bumped, otherwise another major bump required.

[falkecarlsen]