Add experimental compile/generate handling (#49491)

This implements experimental`compile`/`generate` commands to allow
separate the compilation and static generation steps of the build.
Initially this is for testing and follows same experimental rules which
aren't covered by semver.

Note: most changes are just spacing changes so disabling that in the
diff will be helpful.

x-ref: [initial
thread](https://www.notion.so/vercel/bbdbba2016ed464eb5263d325fecc02f?pvs=4)
x-ref: [slack
thread](https://vercel.slack.com/archives/C03DQ3QFV7C/p1676591260523589?thread_ts=1676554904.334789&cid=C03DQ3QFV7C)

packages/next/src/bin/next.ts

@@ -62,6 +62,11 @@ if (!foundCommand && args['--help']) {
 }
 
 const command = foundCommand ? args._[0] : defaultCommand
+
+if (['experimental-compile', 'experimental-generate'].includes(command)) {
+  args._.push('--build-mode', command)
+}
+
 const forwardedArgs = foundCommand ? args._.slice(1) : args._
 
 if (args['--inspect'])