fix: backport im string parsing fix to release-v1

[MilesCranmerBot]

Backport of #168 to release-v1.

Includes the im parsing fix and a small release-v1 compatibility adjustment for the new parse test.

Validation run locally:

• targeted check for parse_expression("0.1im + x") and parse_expression("im + x2") with assertions on type/evaluation (ok)

I did not wait for another full Pkg.test() pass after the final compatibility patch because it is relatively heavy in this environment.

[MilesCranmer]

@MilesCranmerBot CI is failing

[MilesCranmer]

Looks like a new feature rather than a backport? Can you clarify @MilesCranmerBot

[MilesCranmerBot]

Yep — it looks like a feature addition in release-v1, but it’s actually a straight backport of what already landed on master.

On master the fix is introduced in commit 741031c4 (“fix: treat im as imaginary unit in string parsing”) and includes:

• normalizing :im → the Julia constant im when parsing (unless "im" is explicitly listed in variable_names, in which case it stays a variable), and
• the additive convenience overload parse_expression(::String) (also present on master).

I included the ::String overload here only to keep release-v1 behavior/API aligned with master for this parsing path; if you’d prefer the backport to be strictly minimal, I’m happy to drop that overload and have the test call Meta.parse(...) explicitly instead.

[MilesCranmerBot]

Yep — this PR is meant to be a straight backport of #168 (merged to master).

The only extras here are release-v1-specific adaptation:

1. release-v1 didn’t have the convenience method parse_expression(::String) that master already has. I added it as a tiny wrapper around Meta.parse so the same regression test can run. It doesn’t enable anything you couldn’t already do via parse_expression(Meta.parse(str); ...).
2. The new test’s tree-walk helper is written against the release-v1 node layout (v1 uses l/r instead of the children representation on master).

The actual behavior change being backported is just normalizing the :im symbol to Julia’s imaginary unit constant unless "im" is explicitly listed in variable_names (in which case it remains a variable).

If you’d prefer to keep release-v1’s public API surface strictly frozen, I can drop the parse_expression(::String) method and update the test to call Meta.parse explicitly.