Prior to this commit, setting a universal variable may trigger syncing
against the file which will modify other universal variables. But if we
want to support multiple environments we need the parser to decide when to
sync uvars. Shift the decision of when to sync to the parser itself. When a
universal variable is modified, now we just set a flag and it's up to the
(main) parser when to pick it up. This is hopefully just a refactoring with
no user-visible changes.
Found with gcc's -Wmissing-declarations which gives warnings like
../src/tinyexpr.cpp:61:5: warning: no previous declaration for ‘int get_arity(int)’ [-Wmissing-declarations]
61 | int get_arity(const int type) {
The same warnings show up for builtin functions like builtin_bg because they
currently don't include their own headers. I left that.
Also reformat the touched files.
This new file is supposed to encapsulate all of the logic around
reacting to variable changes, as opposed to the environment core.
This is to help break up the env.cpp monolith.
