[Infra] [Minor] Address build system deprecation warnings

[addyosmani]

In particular, see the deprecation warnings around options.mainFields:

warning "microbundle > rollup-plugin-typescript2@0.19.3" has incorrect peer dependency "rollup@>=0.68.0".
[4/4] 🔨  Building fresh packages...
warning Your current version of Yarn is out of date. The latest version is "1.22.5", while you're on "1.22.4".
info To upgrade, run the following command:
$ curl --compressed -o- -L https://yarnpkg.com/install.sh | bash
$ yarn run -s build
node-resolve: setting options.module is deprecated, please override options.mainFields instead
node-resolve: setting options.jsnext is deprecated, please override options.mainFields instead
node-resolve: setting options.module is deprecated, please override options.mainFields instead
node-resolve: setting options.jsnext is deprecated, please override options.mainFields instead
node-resolve: setting options.module is deprecated, please override options.mainFields instead
node-resolve: setting options.jsnext is deprecated, please override options.mainFields instead
Build "quicklink" to dist:
        992 B: quicklink.js.gz
        846 B: quicklink.js.br
        995 B: quicklink.mjs.gz
        846 B: quicklink.mjs.br
       1061 B: quicklink.umd.js.gz
        906 B: quicklink.umd.js.br
✨  Done in 39.70s.

[addyosmani]

Upgrading to latest microbundle addresses this issue, but I'm seeing some local test failures that will require further debugging. Will do so before landing a fix here.

[XhmikosR]

This is fixed on master, feel free to tweak the microbundle properties later :)

[addyosmani]

Thank you!