fix: check functions with implicit return types

[JasonWeinzierl]

Fixes finnish, no-finnish, and no-misused-observables failing to check functions with implicit return types.

Resolves #57

[github-actions]

LCOV of commit 54d222b during .github/workflows/ci.yml #502

Summary coverage rate:
  lines......: 96.7% (4342 of 4488 lines)
  functions..: 96.1% (270 of 281 functions)
  branches...: no data found

Files changed coverage rate:
                                         |Lines       |Functions  |Branches    
  Filename                               |Rate     Num|Rate    Num|Rate     Num
  =============================================================================
  src/etc/get-type-services.ts           |12.0%     83| 0.0%    10|    -      0