fix(job): Deduplicate env var names on Windows

Windows handles environment variable names case-insensitively (i.e.,
%Path% can be accessed via %PATH%).  Since nvim explicitly creates the
environment block for jobs, we need to ensure it doesn't contain
duplicate names with different cases (e.g., both %Path% and %PATH%) as
that can confuse processes using the environment.

`f_environ()` now converts all env var names to upper-case on Windows,
since it does duplicate detection and also provides the base environment
for jobs.  Similarly, `create_environment()` converts any env var names
supplied in the job opts to upper-case so they can be compared against
what we get from `f_environ()`.

Closes #14541

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