[RFC]: add various physical constants (tracking issue)

### Description

This RFC proposes adding various physical constants.
The purpose of this issue is to serve as a tracking issue for adding various physical constants.

List of physical constants to be added:
- [ ] `c` (speed of light in vacuum)
- [ ] `speed_of_light` (speed of light in vacuum)
- [ ] `mu_0` (vacuum magnetic permeability)
- [ ] `epsilon_0` (vacuum electric permittivity)
- [ ] `h` (Planck constant)
- [ ] `Planck` (Planck constant)
- [ ] `hbar` (`h / (2 * pi)`)
- [ ] `G` (Newtonian constant of gravitation)
- [ ] `gravitational_constant` (Newtonian constant of gravitation)
- [ ] `g` (standard acceleration of gravity)
- [ ] `e` (elementary charge)
- [ ] `elementary_charge` (elementary charge)
- [ ] `R` (molar gas constant)
- [ ] `gas_constant` (molar gas constant)
- [ ] `alpha` (fine-structure constant)
- [ ] `fine_structure` (fine-structure constant)
- [ ] `N_A` (Avogadro constant)
- [ ] `Avogadro` (Avogadro constant)
- [ ] `k` (Boltzmann constant)
- [ ] `Boltzmann` (Boltzmann constant)
- [ ] `sigma` (Stefan-Boltzmann constant)
- [ ] `Stefan_Boltzmann` (Stefan-Boltzmann constant)
- [ ] `Wien` (Wien displacement law constant)
- [ ] `Rydberg` (Rydberg constant)
- [ ] `m_e` (electron mass)
- [ ] `electron_mass` (electron mass)
- [ ] `m_p` (proton mass)
- [ ] `proton_mass` (proton mass)
- [ ] `m_n` (neutron mass)
- [ ] `neutron_mass` (neutron mass)

### Related Issues

No.

### Questions

No.

### Other

No.

### Checklist

- [X] I have read and understood the [Code of Conduct](https://github.com/stdlib-js/stdlib/blob/develop/CODE_OF_CONDUCT.md).
- [X] Searched for existing issues and pull requests.
- [X] The issue name begins with `RFC:`.

This is a broad tracking RFC; no source file, test, entry point, or implementation convention is named. Start by locating the project's existing mathematical constants and reviewing related contribution conventions, then define the scope and validation needed for the full checklist before treating the work as done.

Written by the indexing model from the issue text.