* Allow set_environment_variables to set the SHELL.
Previously, we'd always set it to the shell from the password
database. Now we do that by default, but if set_environment_variables
has been used, we'll preserve that value for the environment
of the to-be-spawned process
* Clarify the docs:
* Remove the confusing version dependent sections that started
with old behavior and rewrite in terms of the behavior that
has been true for the past year
* I've heard from a few people that they tried to change COMSPEC
on Windows and pain ensued. Try to nudge them to read the
next paragraph that tells them what they are actually supposed
to change.
refs: https://github.com/wez/wezterm/issues/4098
refs: https://github.com/wez/wezterm/issues/4168
closes: https://github.com/wez/wezterm/issues/3816
refs: https://github.com/wez/wezterm/issues/3317
Nudge new users towards using this style:
```lua
local config = {}
config.color_scheme = 'Batman'
return config
```
and surface how to write lua modules closer to the main section
on config files. In that lua modules section, nudge towards using
a convention similar to that of the plugin spec described in
this commit: e4ae8a844d
I've had a couple of people report issues like
https://github.com/wez/wezterm/issues/2378 recently, and we can avoid
this class of problem by unsetting SHELL at proces startup, so that's
what we do.
Since the original code was written, Rust grew and stabilized
an interface for specifying argv0 for the child process, so
we can remove the fragile `-l` argument passing.
refs: https://github.com/wez/wezterm/issues/753
