This adds a new realm setting: default_code_block_language.
This PR also adds a new widget to specify a language, which
behaves somewhat differently from other widgets of the same
kind; instead of exposing methods to the whole module, we
just create a single IIFE that handles all the interactions
with the DOM for the widget.
We also move the code for remapping languages to format_code
function since we want to preserve the original language to
decide if we override it using default_code_clock_language.
Fixes#14404.
Anders Kaseorg
Rohitt Vashishtha