Maintenance: test coverage and dropping support for lark < 1.0

All but 20ish lines in blark.transform are covered
The rest of the codebase isn't in such good shape, but I'll get around to it at some point
Dropping support for old versions of lark as the added per-version conditionals in the code are confusing/unnecessary
Some things were cleaned (unused InitializedStructureTypeDeclaration)
Unreachable code was removed
VAR_STAT dataclass was fixed (this was entirely broken)

Another TODO item: add pytest-cov to the GH actions and get coverage reports in PRs

klauer / blark