feat(prefer-root-operators): new rule

[JasonWeinzierl]

Help developers migrate from rxjs/operators to rxjs.

• Handles both imports and exports.
• Will auto-rename any operators that have a different name in rxjs (concat -> concatWith as concat).
• Will not attempt to auto-fix if there's a risk of breaking code. e.g. the partition operator was not moved at all, so we leave that import alone. Ditto for * imports/exports.
• This will likely be added to the recommended config when v1 releases. That change will also include bumping the RxJS peer dependency version to v7.2 instead of v7.0.

Resolves #9 .

Feedback is welcome, especially on the rule's name.

[github-actions]

LCOV of commit fca768c during .github/workflows/ci.yml #110

Summary coverage rate:
  lines......: 96.2% (3628 of 3770 lines)
  functions..: 95.6% (237 of 248 functions)
  branches...: 91.3% (732 of 802 branches)

Files changed coverage rate:
                                         |Lines       |Functions  |Branches    
  Filename                               |Rate     Num|Rate    Num|Rate     Num
  =============================================================================
  src/index.ts                           | 100%    101|    -     0|    -      0
  src/rules/prefer-root-operators.ts     |94.9%    137| 100%    16|90.7%     43