Enforce ASCII in README.md (#513)

This all started because I was going to write a script to autogenerate
the Table of Contents in the root `README.md`, but I noticed that the
`href` for the "Why Codex?" heading was `#whycodex` instead of
`#why-codex`. This piqued my curiosity and it turned out that the space
in "Why Codex?" was not an ASCII space but **U+00A0**, a non-breaking
space, and so GitHub ignored it when generating the `href` for the
heading.

This also meant that when I did a text search for `why codex` in the
`README.md` in VS Code, the "Why Codex" heading did not match because of
the presence of **U+00A0**.

In short, these types of Unicode characters seem like a hazard, so I
decided to introduce this script to flag them, and if desired, to
replace them with "good enough" ASCII equivalents. For now, this only
applies to the root `README.md` file, but I think we should ultimately
apply this across our source code, as well, as we seem to have quite a
lot of non-ASCII Unicode and it's probably going to cause `rg` to miss
things.

Contributions of this PR:

* `./scripts/asciicheck.py`, which takes a list of filepaths and returns
non-zero if any of them contain non-ASCII characters. (Currently, there
is one exception for ✨ aka **U+2728**, though I would like to default to
an empty allowlist and then require all exceptions to be specified as
flags.)
* A `--fix` option that will attempt to rewrite files with violations
using a equivalents from a hardcoded substitution list.
* An update to `ci.yml` to verify `./scripts/asciicheck.py README.md`
succeeds.
* A cleanup of `README.md` using the `--fix` option as well as some
editorial decisions on my part.
* I tried to update the `href`s in the Table of Contents to reflect the
changes in the heading titles. (TIL that if a heading has a character
like `&` surrounded by spaces, it becomes `--` in the generated `href`.)

.github/workflows/ci.yml

@@ -67,3 +67,6 @@ jobs:
 
       - name: Build
         run: pnpm run build
+
+      - name: Ensure README.md contains only ASCII and certain Unicode code points
+        run: ./scripts/asciicheck.py README.md