Move yarn plugins to top level

[philrz]

When digging into the root cause of #2839, we could see in the output of yarn install --inline-builds that this line in the Zed Makefile was failing due to the tmp dir for the Zed repo checkout not having a .git directory. The changes in the move to Monorepo in #2818 ended up causing this problem because the Yarn plugins (such as the "plugin-github" one) have since only been present below apps/zui/.yarn but are actually needed in the top-level .yarn directory since (as @jameskerr educated me) that's where the Yarn workspace is defined.

In addition to fixing the zed -version problem (#2839) this also addresses the equivalent brimcap -version problem that was mentioned in brimdata/brimcap#316 as waiting in line behind #2840. With the changes in this PR, after running yarn, we now see correct version strings.

$ node_modules/zed/dist/zed -version
Version: v1.9.0-10-g08fc271c

$ node_modules/brimcap/build/dist/brimcap -version
Version: v1.4.0-26-gd6a430d

Since I had to locally confirm that all the Zui tests run ok after these changes, I went ahead and advanced the dependency pointers for both Zed and Brimcap as part of this PR. For the Brimcap case, since I also need the fix in brimdata/brimcap#316, right now I'm pointing at the commit for that fix branch. Assuming brimdata/brimcap#316 gets approved and merged first, I'll then advance the Brimcap pointer in this PR to the new tip of Brimcap's main.

Fixes #2839