Issues with the Preact preset linting config

[edmorley]

Spotted whilst working on babel 7.

The @neutrinojs/preact currently depends on eslint-plugin-react:
https://github.com/mozilla-neutrino/neutrino-dev/blob/082515268e3587e065989c787c0bd35b6f6b8dab/packages/preact/package.json#L37

...however it doesn't activate the react plugin (unlike @neutrinojs/react):
https://github.com/mozilla-neutrino/neutrino-dev/blob/082515268e3587e065989c787c0bd35b6f6b8dab/packages/preact/index.js#L50-L57

...meaning that dependency is currently unused.

We should probably activate the plugin and also set the pragma correctly, to prevent false positives. eg:

      plugins: ['react'],
      baseConfig: {
        settings: {
          react: {
            pragma: 'h'
          }
        }
      }