Set `process.env.PATH` based on configuration

Instead of invoking `latexmk` with a full path, we now make sure
that `process.env.PATH` contains the configured `texPath` value.
If the configured path does not contain $PATH at some point in the
string, we show an error message and abort.

Also contains some minor tweaks, and code cleanup.

(Fixes #2)
Merge branch 'set-path-on-activation'