feat: add LitElement to recommended config

[43081j]

Introducing the invalid-extends rule into the best practice config resulted in a larger breaking change than expected: LitElement repos fail to lint without introducing an elementBaseClasses setting.

Since a large chunk of our consumers are lit users, it makes sense to add it to the recommended config so it doesn't need configuring individually.

[43081j]

@keithamus what do you think about this? i'm torn

on one hand, it'd save people time since they can upgrade without many config changes.

on the other, it means error messages in non-lit repos will say things like should extend HTMLElement or LitElement

basically without this it means many consumers will have to introduce a settings object to specify LitElement as a base class

i suppose the ideal is a lit config, but im just trying to think of a way to make the migration a bit easier (from 1.x to 2.x)

[keithamus]

I think the PR is fine as is but I also wonder - could we use heuristics to determine if its a lit project, e.g. reading from package.json?

[43081j]

i did wonder that too, maybe we could get away with just require.resolve('lit')? it isn't exact, in that it could be a deep dependency, but maybe that's good enough?

otherwise, we could look for a package.json and see if it contains lit (keep in mind eslint is sync, so we would be using readFileSync etc)

edit: i've updated this to use the require.resolve method, let me know if you think that'll be fine

[keithamus]

I think this is great!