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>

README.md

@@ -2,6 +2,26 @@
 
 Sequel to pocomath as a prototype for mathjs overhaul
 
+### Set up
+
+1. Install the `pnpm` package manager on your machine.
+2. Clone the repository `https://code.studioinfinity.org/glen/nanomath.git`
+   and change directory into its top level.
+3. `pnpm install`
+4. `pnpm test` runs the tests.
+
+## Design philosophy
+
+Note the rest of this README lays out the initial structure and design
+goals for the nanomath prototype, and remains a general guide for its
+development. Some specific details have been adjusted, and this document
+has not necessarily been kept up-to-date. See the
+[project wiki](https://code.studioinfinity.org/glen/nanomath/wiki) for
+the most recent details; where the wiki and this README differ, the wiki
+is presumed to be authoritative.
+
+### Themes
+
 The main themes of nanomath as compared to its predecessor are:
 
 1. Settle on a code organization, rather than merely demonstrating that
@@ -29,7 +49,7 @@ The main themes of nanomath as compared to its predecessor are:
    ported if a decision were taken to return to something more highly
    typescript-integrated.
 
-## Code organization
+### Code organization
 
 The pocomath prototype showed that we could have one-file-per-implementation, or
 multiple-files-per-implementation, and that we could group implementations by