[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:`.