Add "no-fallthrough" to base rules

[benjaminpearson]

Ensure switch statements contain a break, return or throw in each case. I believe most of the time engineers would intend for a case statement to not fallthrough to the next statement. Thus this eslint rule (which is part of eslint:recommended - see source) I'd say is a sensible default.

Rule: https://eslint.org/docs/latest/rules/no-fallthrough

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eslint-config-seek	Minor

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[askoufis]

Do we want to get this out with #140?

[72636c]

For posterity, just noting that this rule can report false positives because it doesn't have type information to prove exhaustiveness: typescript-eslint/typescript-eslint#3455

I suspect it's still a net positive and we should keep it for the general case.