Add comprehensive tests

[aluttik]

There's a bunch more things that need to be tested here.

To name a few:

• test that errors are raised when absolute includes point to a path that doesn't exist.
• test blocks that have special inner contexts like map or geo
• test that errors are raised when the wrong number of args are given for a known directive.
• test that no directive errors are raised for unknown directives.
• test that errors are raised when directives are in the wrong context
• test that args can't start with variable expansion syntax (${...})
• test that delimiters are treated like arguments when they are quoted (e.g. '{', ";", etc)