Introduce introspect.EnvVar which tracks all the environment variables
that are observed by the compiler, so that we can print them with `zig
env`.
The `zig env` command now prints both the resolved values as well as all
the possibly observed environment variables.
The current json output is not very convenient to use from the shell or
a Zig program.
An example using the shell: `zig env | jq -r .lib_dir`.
Remove the json output, and instead use the POSIX shell syntax:
name="value"
Additionally, when args is not empty, assume each argument is the
environment variable name and print the associated value.
Unrecognized environment variables are ignored.
The new output format has been copied from `go env`, with the difference
that `go env` uses OS specific syntax for windows and plan9.
Define the environment variables in a single place, in order to avoid
possible bugs.
