`letenv!` is a layover from the days before
`with-environment-variables` (introduced in 28.x), and it remained
afterwards because I preferred the shorter name. From v3 and onward,
Doom's core will be put on a diet which, among other things, will
include culling redundant or superfluous functions/macros like this one.
BREAKING CHANGE: This restructures Doom's core in an effort to slim it
down and partially mirror architectural changes coming in v3. This is
part 2 of 3 commits (part 1 being 1590434), done to facilitate a change
in part 3 that will introduce a new `doom!` syntax for pulling
third-party module libraries from remote sources (similar to `package!`
statements). I am backporting this from V3 so I can move our modules out
into separate repos sooner than later, so development on modules can
continue separately without interfering with v3's roll out.
Though this is labeled a breaking change, it shouldn't affect most users
except those few tinkering directly with Doom's internals.
Ref: 15904349cf
