LSP: add call inlays for complex literals

[DanilaFe]

Closes #25115.

This just involves adding some smarter AST inspection to detect what we want to treat like "literals" for the purposes of call inlays. Specifically, I've opted for the following:

1. Complex number literals should only have 'two' components; in the same way that 1+1 is not a literal, neither is 1 + 1i + 1. On the other hand, 1 + 1i is a literal.
2. The numbers can be in any order: 1 + 1i or 1i + 1. This is a bit liberal, but I think it's reasonable.
3. Double negation is not allowed, neither for real/imaginary numbers nor complex literals.

• So, -4 is a "literal", but - -4 is not.
• Similarly, 4 - (-i) is not a literal.

I've un-xfailed the test that locked this down, and added a separate test for negative numbers.

Reviewed by @jabraham17 -- thanks!

Testing

• make test-cls