Adds @readonly annotation support to mobx_codegen

[Ascenio]

Continuation of #647.
Replacement of #630.

[codecov]

Codecov Report

Merging #649 (d754e5f) into null_safety (16961a0) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           null_safety     #649      +/-   ##
===============================================
+ Coverage        99.16%   99.17%   +0.01%     
===============================================
  Files               52       53       +1     
  Lines             1793     1827      +34     
===============================================
+ Hits              1778     1812      +34     
  Misses              15       15              

Flag	Coverage Δ
flutter_mobx	100.00% <ø> (ø)
mobx	99.14% <100.00%> (+0.01%)	⬆️
mobx_codegen	100.00% <100.00%> (ø)

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Impacted Files	Coverage Δ
mobx/lib/src/api/annotations.dart	75.00% <ø> (ø)
mobx_codegen/lib/src/errors.dart	100.00% <100.00%> (ø)
mobx_codegen/lib/src/store_class_visitor.dart	100.00% <100.00%> (ø)
mobx_codegen/lib/src/template/observable.dart	100.00% <100.00%> (ø)
...egen/lib/src/utils/non_private_name_extension.dart	100.00% <100.00%> (ø)

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[pavanpodila]

Now that the branch is merged into master, lets make the PR into master

[Ascenio]

So, should I make another to master? I thought you could change the target branch of the PR.

[Ascenio]

Oh, I can too, that's nice, didn't know about that.

[pavanpodila]

@Ascenio, I think you should also create a PR for the docs and describe an example where @readonly would be useful.

[pavanpodila]

@all-contributors add @Ascenio for code

[allcontributors]

@pavanpodila

I've put up a pull request to add @Ascenio! 🎉