test: Set up a mocha test harness (#5)

We use [mocha](https://mochajs.org/) as the test framework, as it is
  the tool used by mathjs and we would like to make tests as similar
  as possible. However, to tighten the linkage between source code and
  tests, we adopt a somewhat different file organization: unit tests
  for a given source file `blah/foo.js` are in `blah/__test__/foo.spec.js`.

  To run all unit tests, execute the script `pnpm test`.

  Resolves #3.

Reviewed-on: glen/nanomath#5
Co-authored-by: Glen Whitney <glen@studioinfinity.org>
Co-committed-by: Glen Whitney <glen@studioinfinity.org>

package.json5

@@ -3,13 +3,19 @@
   version: '1.0.0',
   description: 'Sequel to mathjs as prototype for mathjs overhaul',
   scripts: {
-    test: 'echo "Error: no test specified" && exit 1'
+    test: 'mocha ./**/*.spec.js',
   },
-  keywords: ['math', 'algebra'],
+  keywords: [
+    'math',
+    'algebra',
+  ],
   author: 'Glen Whitney',
   license: 'Apache 2.0',
   repository: {
     type: 'git',
     url: 'https://code.studioinfinity.org/glen/nanomath.git',
   },
+  devDependencies: {
+    mocha: '^11.1.0',
+  },
 }