ConTeXt: various fixes, support some embeddings and add some missing title macros

Things this fixes, most examples here are actually in the test file itself

![1](/uploads/d53d73d26e710ac27790a47de9e5e026/1.png)

![2](/uploads/0af5045017d42bab9feae8845f98f352/2.png)

![3](/uploads/6e65520c551aaf01844f7b5abb238c51/3.png)

![4](/uploads/e645dafeed95769a55192295c9fca123/4.png)

![5](/uploads/cb2526098057d98afd25921125381add/5.png)

![6](/uploads/028ce06ef2f57acb97f41d9d5fa19c3c/6.png)


This adds support for highlighting embedded Lua, Metapost, and XML, which work by default in ConTeXt.

... Adding support for other languages is more complicated because the name of the commands are user defined ...


Also recolor Metafont/Metapost using more default definitions because it looked bad on dark mode.

Before: ![mpBefore](/uploads/09f6fec80697f3c0fa3a92b8242cc3b3/mpBefore.png){width=20%}
After: ![mpAfter](/uploads/c1e9edc797214b2bc5a78fc6540decf6/mpAfter.png){width=20%}