ref(nextjs): Move config code to SDK and fix next version bug

[lobsterkatie]

Two changes here:

• Cramer reported running into trouble because he hadn't specified the next version in his project's package.json exactly, and as a result, the version we copy over to our plugin didn't match what next was expecting. ('^10.1.3' didn't match ''10.1.3', and neither did 10.x.) To avoid such problems, the version number we copy over is now drawn from the version value in next's package.json rather than the dependencies.next value in the project's package.json.

• He also mentioned it would be nice to abstract away as much of the boilerplate config code as we could, and since this was something we as a team had already identified as something we'd like to do, I went ahead and did it. Also, in order for it to be possible to use SDK code in next.config.js (which is vanilla JS), it was necessary to add an ES5 build and a main entry in package.json.

A word of caution when testing this - if you pull the branch and use your linked packages in a test app, it will fail, because the relative paths will anchor themselves in your repo rather than your test app. My solution has been to leave everything linked, and then mv node_modules/@sentry node_modules/@sentry-linked && cp -r node_modules/@sentry-linked node_modules/@sentry, which will create non-sym-linked versions of all of the packages. (Fun fact, using Duplicate in the Finder - I know, I know, it's sacrilege, but I got lazy - just gives you another set of sym-links, not a copy of those links' targets.)

This PR is in coordination with this PR in the wizard and this commit in docs.

[github-actions]

size-limit report

Path	Size
@sentry/browser - CDN Bundle (gzipped)	20.6 KB (0%)
@sentry/browser - Webpack	21.48 KB (0%)
@sentry/react - Webpack	21.52 KB (0%)
@sentry/browser + @sentry/tracing - CDN Bundle (gzipped)	27.82 KB (+0.01% 🔺)