build: Set up jasmine for unit testing

build: Set up jasmine for unit testing

Jasmine[0] is a popular testing framework for javascript, which
gjs itself has used in a heavily customized form for its own
unit tests for years.

It's far superior to the very rudimentary `jsUnit` module that
is still included with gjs, and the jasmine-gjs project provides
a general-purpose wrapper for gjs apps.

Set up the necessary build infrastructure to run unit tests
through jasmine. That allows for existing unit tests to be
ported one-by-one.

[0] https://jasmine.github.io/index.html

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3164>