feat(units): add an expression parser

[HaoZeke]

Closes #154.

Replaced the per-quantity lookup tables with a Shunting-Yard expression parser
that works on arbitrary compound unit strings in the spirit of lumol.

"kJ/mol/A^2"  -->  tokenize  -->  shunting-yard  -->  AST  -->  eval
                   [kJ,/,mol,     [kJ,mol,/,         tree     {factor, dim}
                    /,A,^,2]       A,2,^,/]

Each token resolves to an SI conversion factor and a 5-element dimension vector
[L, T, M, Q, Theta]. The parser composes these through multiplication, division,
and exponentiation. Conversion factor between two expressions = ratio of their
SI factors after verifying dimension equality.

API changes

| Before (3-arg)                                  | After (2-arg)                                    |
|-------------------------------------------------+--------------------------------------------------|
| ~unit_conversion_factor("energy", "eV", "meV")~   | ~unit_conversion_factor("eV", "meV")~              |
| ~unit_conversion_factor("force", "eV/A", "eV/A")~ | ~unit_conversion_factor("eV/A", "eV/A")~           |
| Not possible                                    | ~unit_conversion_factor("(eV*u)^(1/2)", "u*A/fs")~ |

Expression syntax

Operators: * (multiply), / (divide), ^ (power), () (grouping).
Whitespace ignored. Case-insensitive. Numeric literals allowed in exponents.
Fractional exponents via parenthesized division: ^(1/2).

Token table

Single flat unordered_map with 30+ entries covering length (angstrom, bohr, nm,
m, cm, mm, um), energy (eV, meV, hartree, ry, joule, kcal, kJ), time (fs, ps),
mass (u, kg, g, electron_mass), charge (e, coulomb), dimensionless (mol), and
derived (hbar).

Notes

kelvin is NOT in the token table because temperature conversions between
offset-based scales (Celsius, Fahrenheit) are non-multiplicative.
DIM_TEMPERATURE exists as dimension [0,0,0,0,1] for potential future use but
no tokens currently carry it. (maybe once we do an API break, can revisit during mini-metatomic)

Contributor (creator of pull-request) checklist

• Tests updated (for new features and bugfixes)?
• Documentation updated (for new features)?
• Issue referenced (for PRs that solve an issue)?

Reviewer checklist

• CHANGELOG updated with public API or any other important changes?

[GardevoirX]

Thanks a lot! I think it would be better if we can use the new functionality to check if the quantity and unit match, when initializing ModelOutput here
https://github.com/HaoZeke/metatomic/blob/0fe0ef73e82b089811278472acba56267578b80c/metatomic-torch/include/metatomic/torch/model.hpp#L48-L61

[GardevoirX]

LGTM, love it!