Rename lex section testdata
test_lex_sections.dat: Rewrite

* Add proper name.
* Rewrite expected object output in json notation.
test_lex_sections.dat: Change #tree to #expect
test_lex_sections.dat: Rewrite JSON objects
lex.go: Make item fields exportable
lex.go: Change isSectionAdorment argument to l.peek()
lex_test.go: Rewrite test handling

The tests for the scanner now pulls the data from test_lex_section.dat which
organizes each test by name, description, data, and expect. The expect section
is a json representation of the lexed items for comparison.
lex.go: Add systemMessageLevel type