ESM: Fix importing `index.js` files by folder name

Description of the Change

This is a fix for #4665, where running with -r ts-node/register or similar flags doesn't work.

The reason this fails is because Node cannot resolve node_modules/ts-node/register/index.js as an ESM module. The returned error has a code of 'ERR_UNSUPPORTED_DIR_IMPORT', which should trigger the require fall-back logic, but it doesn't because that error code is not on the list.

Alternate Designs

Mocha v8 used a much simpler try / catch approach that worked reliably. Mocha v9 tries to be more picky, which is good, but we need to include all the edge cases for this to work smoothly.

Why should this be in core?

It's a bug-fix for a pretty serious v9 regression.

Benefits

mocha -r ts-node/register works again! Or mocha -r sucrase/register, if you prefer that one.

Possible Drawbacks

I can't think of any - it's a pretty obvious fix.

Applicable issues

#4665

This is a minor patch fix for a pretty common issue, so it would be good to release as v9.0.1 -> v9.0.2.