fix: detect if `@sanity/browserslist-config` can't be resolved

[stipsan]

Fixes #1397 but unsure if it can land since findPackageJSON isn't well supported yet. I initially tried with createRequire, but I couldn't get it to fail when running from within @sanity/pkg-utils, it has to detect resolution errors from cwd. Since @sanity/pkg-utils has a direct dependency on @sanity/browserslist-config there's always going to be ./node_modules/@sanity/pkg-utils/node_modules/@sanity/browserslist-config or ./node_modules/@sanity/browserslist-config. It needs to detect the ./node_modules/@sanity/pkg-utils/node_modules/@sanity/browserslist-config case specifically, or rather, that ./node_modules/@sanity/browserslist-config is "missing" and thus "browserslist": "extends @sanity/browserslist-config" won't have an effect in other tooling than @sanity/pkg-utils itself.

I put "missing" in quotes, as it's not that simple. ./node_modules/@sanity/browserslist-config might be missing and its fine, if ../../node_modules/@sanity/browserslist-config does. That's why I'm trying to rely on core node resolution APIs so that we can know for a fact wether it's failing to resolve or not.