Add rule `no-misused-observables`

[JasonWeinzierl]

We already have no-ignored-observable which is similar to typescript-eslint's no-floating-promises. typescript-eslint has a counterpart rule, no-misused-promises which could be useful for Observables too.

@typescript-eslint/no-unnecessary-condition already can handle the checksConditionals portion, so just do checksVoidReturn and checksSpreads.

[JasonWeinzierl]

Also consider renaming no-ignored-observable to no-floating-observables to be more clear about what the rule does (and align with typescript-eslint).