feat(gee): treat if-then as if-then-else w/ empty else block

feat(readme): add instructions for running lab 3

refactor(clippy): address clippy lints

feat(lab3): add initial implementation of the Gee programming language parser

feat(parser): add terminal and token_span methods to ParseTree

coerce empty derivations to epsilon

refactor(parser): use global error type for parser

gplv3 license

feat(grammar): add tests for an LL(1) "elevator grammar"

feat(scanner): support unicode

refactor: remove unnecessary Debug derives from scanner rule types

add a test w/ a non-ll grammar, and run cargo fmt --all

refactor: improve grammar error handling and LL(1) checks

feat: implement parser binary (lab2)

fix: update grammar rules and label macros and comments as whitespace so they are skipped