feat(no-misused-observables): new rule

[JasonWeinzierl]

New rule based on @typescript-eslint/no-misused-promises. Ensures you don't return an Observable where void is expected, or try to spread an Observable. This rule is added to the strict configuration.

Resolves #43 .

Note: checksVoidReturn is noticeably slow, specifically stemming from the arguments, properties, and variables sub-options. It's still faster than @typescript-eslint/no-deprecated and some import-x rules, but it's in the top 10 with TIMING=1 enabled. But since the implementation is similar to @typescript-eslint/no-misused-promises which has similar performance, we'll allow it.

[github-actions]

LCOV of commit a8063fc during .github/workflows/ci.yml #174

Summary coverage rate:
  lines......: 96.6% (4190 of 4336 lines)
  functions..: 96.0% (267 of 278 functions)
  branches...: 91.0% (918 of 1009 branches)

Files changed coverage rate:
                                         |Lines       |Functions  |Branches    
  Filename                               |Rate     Num|Rate    Num|Rate     Num
  =============================================================================
  src/configs/strict.ts                  | 100%     40| 100%     1| 100%      1
  src/etc/get-type-services.ts           | 100%     70| 100%    10|96.0%     25
  src/etc/is.ts                          |74.6%    118|69.7%    33| 100%     23
  src/index.ts                           | 100%    105|    -     0|    -      0
  src/rules/no-misused-observables.ts    |99.1%    427| 100%    18|89.8%    177