Add avoid non null assertion rule

[incendial]

What is the purpose of this pull request? (put an "X" next to an item)

[ ] Documentation update
[ ] Bug fix
[X] New rule
[ ] Changes an existing rule
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

[codecov-commenter]

Codecov Report

Merging #285 (e5a30ff) into master (4b61e8b) will increase coverage by 0.13%.
The diff coverage is 91.66%.

❗ Current head e5a30ff differs from pull request most recent head 9dbbe53. Consider uploading reports for the commit 9dbbe53 to get more accurate results

@@            Coverage Diff             @@
##           master     #285      +/-   ##
==========================================
+ Coverage   73.01%   73.14%   +0.13%     
==========================================
  Files         108      109       +1     
  Lines        3272     3296      +24     
==========================================
+ Hits         2389     2411      +22     
- Misses        883      885       +2     

Impacted Files	Coverage Δ
lib/src/obsoleted/rules_factory.dart	76.19% <0.00%> (-3.81%)	⬇️
...obsoleted/rules/avoid_non_null_assertion_rule.dart	100.00% <100.00%> (ø)

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[roman-petrov]

Just got an email notification about this PR and I think it is great rule! I already seen a feature request asking to implement exactly the same in dart linter repository

Lint rule for ! postfix bang/exclamation and late null safety language features

[incendial]

Yeah, I've actually seen this issue and decided to add this feature. Unfortunately there is no rule requests in our repo yet, hope this will change in the future.

[dkrutskikh]

@incendial please remove withCommentOrMetadata: true,

[incendial]

Done.