Convert @blueprintjs/tslint-config to eslint

[johnwiseheart]

tslint is deprecated.

I propose adding a new package @blueprintjs/eslint-config which is a duplicate of @blueprintjs/tslint-config but converted to eslint. We can keep both for the time being (its rare that they are touched) and when blueprint next major versions, we can remove the tslint ones.

Probably at the same time, we'd want to migrate the linting of blueprint itself to use eslint.

Thoughts?

[adidahiya]

Done