Introduce value_or() for std::map #3444

Inspired by #1571.

There a template function value_or_default() is used to replace the ubiquitous design pattern of checking for the presence of a key in a std::map, and yielding its value if present or some default value if absent. This is a frequent and annoying pattern so would be nice to have a clean substitute. While it seems std::map hasn't inherited such a functionality yet in the language specification, std::optional sets a precedent with value_or().

Could make an attempt at a regular expression for check_syntax that could find all such instances. An LLM could likely recognise them but it's both an expensive sweep of the code base and would not protect against regression.