Enable `explicit-function-return-type` ESLint rule

[andyleejordan]

We should add:

    "@typescript-eslint/explicit-function-return-type": [
      "error"
    ],

but enabling it currently results in 96 problems...my bad! That's a tech debt day item to fix for sure.

Originally posted by @andschwa in #3903 (comment)

[andyleejordan]

https://typescript-eslint.io/rules/explicit-function-return-type/