Migrate package manager to Yarn Modern (#628)

We were using Yarn Classic. While this is not deprecated, Yarn is moving
in a different direction with Yarn Modern (read more
[here](https://yarnpkg.com/migration/overview)) and we should align with
that. The migration mainly requires rebuilding our lockfile. We will
still use `node_modules` for now, though in the future we may want to
adopt the new [Plug'n'Play model](https://yarnpkg.com/features/pnp).

.gitignore

@@ -22,3 +22,12 @@ test/
 .vscode/
 make/
 waveterm-builds.zip
+
+# Yarn Modern
+.pnp.*
+.yarn/*
+!.yarn/patches
+!.yarn/plugins
+!.yarn/releases
+!.yarn/sdks
+!.yarn/versions