Resolve postcss-focus-within to v4

[ayushn21]

There's a bug in postcss-focus-within v3 which has been fixed in v4; but postcss-present-env still relies on v3. There's a PR open to update it for over a year: csstools/postcss-preset-env#172

To provide a better developer experience, I reckon it's best to fix this within Bridgetown by explicitly resolving this package to v4. I've set the resolution in our default package.json. I don't think there's much point in adding this resolution just when the --use-postcss flag is added as it doesn't really do any harm and I don't want to litter the file with multiple ifs.

Also, this approach means we don't have to set the resolution in our bridgetown webpack enable-postcss command which makes life easier as the yarn set resolution command only exists in Yarn 2.

Fixes #356

[ayushn21]

Hey @jaredcwhite, could you please take a quick peek at this so we can merge it? Thanks! :)