[RFC] LaTeX command lookup for unicode characters.

Have tested on `latex_symbols.jl` file in base. Nearly all characters are recognised correctly apart from:

* `\del` interpreted as `\nabla`
* `\Elzxh` interpreted as `\hbar`
* `\surd` interpreted as `\sqrt`

Some causal timings of the doc lookups shows that `texcommand` has negligible impact compared to `thingorfunc` (~90x faster). As such I've not tried to combine both lookups into a single one since it overcomplicated the code when I tried &ndash; perhaps I was missing something obvious.

 The `LaTeX command for ...` message might be a bit too wordy.