Publish all packages as ESM-only, via package.json exports by eemeli · Pull Request #653

Publish all packages as ESM-only, via package.json exports by eemeli · Pull Request #653 · projectfluent/fluent.js

~~Needs rebasing once #652 lands.~~

Closes #435

We should not need to publish the packages any more as anything other than ESM, and with package.json exports we ought to more clearly delimit what the public APIs of each package are.

Also moves the tools' dependencie sto tools/package.json, and replaces the fluent-gecko/ workspace with a Rollup build in fluent-react/. The FluentSyntax bundle of fluent-gecko/ is no longer used.