Improve statement parsing: clear diagnostics, paren validation

[has9sayed]

Enhance parse_statement with descriptive error messages (using pyparsing labels and the - operator), automatic balanced-parenthesis checking, and full support for # inside trailing comments. This replaces generic parsing failures with clear, caret-based error diagnostics.

For the wrong input v = (5 + 3 now it gives -

ERROR MSG:
Unbalanced parentheses in expression: '(5 + 3'
v = (5 + 3
    ^^^^^^

[mstimberg]

Thanks, this looks generally good to me! While testing, I noticed a small issue that was already present in the previous code, but it would make sense to fix it here as well. Statements like x = (missing RHS) fail parsing (as they should), but x = # missing expression will parse fine, using the comment as the expression.I think we could catch this quite easily here as well.

[mstimberg]

Oh, only just realized that the test suite is failing. This is actually a silly issue, there is a test with a missing identifier (testing whether this is detected) which happens to also have a syntax error, i.e. unbalanced parentheses… Since these are now caught first, the error type is different. Please fix the unbalanced parentheses in that test:

brian2/brian2/tests/test_synapses.py

Lines 3664 to 3673 in a5eccc2

	def test_missing_lastupdate_error_run_regularly():
	G = NeuronGroup(1, "v : 1")
	S = Synapses(G, G)
	S.connect()
	S.run_regularly("v += exp(-lastupdate/dt")
	with pytest.raises(BrianObjectException) as exc:
	run(0 * ms)
	assert exc_isinstance(exc, KeyError)
	assert "lastupdate = t" in str(exc.value.__cause__)
	assert "lastupdate : second" in str(exc.value.__cause__)