Allow feature flags to be used in complex `if` conditions

[nolanlawson]

Currently we can do this:

import features from '@lwc/features'

if (features.ENABLE_SOMETHING) {
  /* ... */ 
}

... but we can't do this:

import features from '@lwc/features'

if (features.ENABLE_SOMETHING || someOtherCondition) {
  /* ... */ 
}

The reason is that the @lwc/features Babel plugin is pretty specific about what it allows in the if() condition:

https://github.com/salesforce/lwc/blob/1deff061eb2c061327b89f734d1857e17a3ce471/packages/%40lwc/features/src/babel-plugin/index.js#L64-L78

We should fix this so that we can use feature flags everywhere and have them "just work." Or at least work in slightly more complex if conditions.