fix(eslint-plugin): support object types and as expressions [explicit-func-return-type]

[bradzacher]

Fixes #408

Adds support for 3 cases:

let asTyped = (() => '') as () => string;

interface ObjectType {
  foo(): number;
}
let objectProp: ObjectType = {
  foo: () => 1,
};

interface ObjectType {
  foo(): number;
}
let asObjectProp = {
  foo: () => 1,
} as ObjectType;

[codecov]

Codecov Report

Merging #459 into master will decrease coverage by 0.05%.
The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master     #459      +/-   ##
==========================================
- Coverage   97.14%   97.09%   -0.06%     
==========================================
  Files          74       74              
  Lines        2875     2891      +16     
  Branches      473      479       +6     
==========================================
+ Hits         2793     2807      +14     
- Misses         49       51       +2     
  Partials       33       33

Impacted Files	Coverage Δ
...-plugin/src/rules/explicit-function-return-type.ts	93.93% <87.5%> (-6.07%)	⬇️