utf8 characters in M2 and Emacs

@pzinn is already working on this.

Since May 18, 2020.

Is there any interest in using utf8 (or even unicode) characters more in Emacs? Recent versions of Emacs as well as most terminal emulators support unicode (not sure since which version). M2 itself also supports utf8 characters for variable names:
```
S = QQ[θ]; -- in Emacs use C-x 8 [Enter] 03B8 [Enter] to type θ
gens S
θ^2
S_0^2
```

Entering them is not super easy, but for instance `S_0` is a an easy shortcut. I don't think allowing `\theta` as variable name would be possible.

As an example of an alternative way, some Sage functions have a [`latex_name`](https://doc.sagemath.org/html/en/reference/calculus/sage/calculus/var.html) option, which just tells Sage to print the LaTeX name of the variable or function when using `latex(variable)`. Example:
```
sage: function('riemann', latex_name="\\mathcal{R}")
riemann
sage: latex(riemann(x))
\mathcal{R}\left(x\right)
```

We could have a similar option so that on viewers supporting utf8 we can print the utf8 character and on viewers supporting LaTeX, for instance in the documentation or the interactive shell, use the LaTeX code and let MathJax or KaTeX do the conversion.

This is somewhat related to #522