feat: modernize parser

[vidhanio]

parser rewritten from the ground-up using syn idioms and more strongly-typed syn structures rather than TokenStreams. passes all test cases flawlessly, and makes the code much easier to understand.

also drops proc-macro-error for a more robust solution (proc-macro2-diagnostics) which doesn't need global state and macros.

all of the edited trybuild test-cases are either adding more information/cases or fixing some bugs.