Add dotenvx to dynamically manage environment variables

The real problem I'm running into is that different versions of node can
support wildly different options in NODE_OPTIONS.

For example `--no-experimental-global-navigator` will be required to make
things work on Node 22, but it isn't recognized at all on older versions,
and throws an error if we try to include it.

I fixed this by adding `scripts/node_options.js` to dynamically decide
which options to use depending on the node version, and dotenvx to set
the enviroment up before running node.

This makes the `npm run` scripts a bit clunky, because they need to
call `dotenvx run --quiet -- ` before doing `node whatever`,
but for now it seems like the least bad option.

This commit also removes `.npmrc` which was great but can only set the
NODE_OPTIONS to a single string.

.env

@@ -0,0 +1 @@
+NODE_OPTIONS=$(node 'scripts/node_options.js')