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>

tests/unit/.eslintrc.yml

@@ -0,0 +1,26 @@
+---
+# SPDX-License-Identifier: MIT OR LGPL-2.0-or-later
+# SPDX-FileCopyrightText: 2024 Florian Müllner <fmuellner@gnome.org>
+rules:
+  jsdoc/require-jsdoc: 'off'
+globals:
+  jasmine: readonly
+  afterAll: readonly
+  afterEach: readonly
+  beforeAll: readonly
+  beforeEach: readonly
+  describe: readonly
+  expect: readonly
+  expectAsync: readonly
+  fail: readonly
+  fdescribe: readonly
+  fit: readonly
+  it: readonly
+  pending: readonly
+  setSpecProperty: readonly
+  setSuiteProperty: readonly
+  spyOn: readonly
+  spyOnAllFunctions: readonly
+  spyOnProperty: readonly
+  xdescribe: readonly
+  xit: readonly