Setup unit tests for lexer

[doriath]

Dependency:

• Setup new lexer in syntax crate #7 Setup new lexer in syntax crate

The goal is to setup unit tests for syntax/src/lexer module (to be created in #7), similar to how parser tests are created (directory with input and expected output).

Example:

# /syntax/test_data/lexer/test.vim
let x = 5

# /syntax/test_data/lexer/test.txt
LET_KW 3 "let"
WHITESPACE 1 " "
IDENT 1 "x"
...