tests: Refactor test_note to use template_test_*

tests: Move parser error tests to its own file

tests: Copy tokens in template_test_invalid ...

This makes debugging easier as our input data isn't consumed.

parse: Add Set node parse code ...

I've taken a different approach this time with my tests by avoiding
decorated functions where I can and adding big upfront documentation
about how things work.

I've deprecated the old decorated functions and hopefully will remove
them during a refactor and cleanup.

parse: Add statement parsing

tests: Use a static parse context for tests

src: Rename syntax to token

parse: Remove SyntaxType ...

Once again I find myself in the pattern of having a useless 'type' value
that just complicates my life. In this case the correct appproach is to
output the AST directly.

tests: Clean up syntax and parse code

tests: Clarify function documentation

tests: Random insert_random to insert_random_within

parse: Handle parent_context in template_parse_invalid

tests: Add template_parse_invalid

tests: Move insert_random to parse.templates

tests: Add a template for parsing valid syntax