Add sample formula metadata to the plan windows

[canismarko]

Per Haven developers meeting on 2025-01-14:

We want to add "sample_formula" as a metadata key to standard plans. To do this, we need a QLineEdit on each plan window.

We also want to include some placeholder text that provides a sample of how to consistently format a chemical formula. How precise should we get with this? Some common Li battery cathodes would be "LiNi0.8Mn0.1Co0.1O2" and "LiNi1/3Mn1/3Co1/3O2". LaTeX formatting? Something else? Too niche to care?

[Cathyhjj]

I think I preferred the "LiNi0.8Mn0.1Co0.1O2" rather than fractions or latex formatting.

Or maybe all of them are fine, instead, we can do a smarter query similar to Material Project? like 'Li-Mn-Ni-O', so this will show up the LNMO no matter which formats ppl use?