fix(amplify-codegen): add hardcoded modelName to Dart

[ragingsquirrel3]

Description of changes

This PR changes Dart modelgen so that instances of the model name are hardcoded rather than taken from toString() on the type, which is not recommended because that name will get minified in some environments (like Flutter web) https://dart.dev/tools/linter-rules#avoid_type_to_string.

Basically, models will have 'Post' hardcoded instead of (Post).toString() or similar.

Issue #, if available

aws-amplify/amplify-flutter#2330

Description of how you validated changes

Checklist

• [x ] PR description included
• [ x] yarn test passes
• [ x] Tests are changed or added
• [ n/a] Relevant documentation is changed or added (and PR referenced)
• [ n/a] Breaking changes to existing customers are released behind a feature flag or major version update
• [ x] Changes are tested using sample applications for all relevant platforms (iOS/android/flutter/Javascript) that use the feature added/modified

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[dnys1]

LGTM!

[codecov-commenter]

Codecov Report

Merging #514 (2d436f0) into main (750ba15) will increase coverage by 2.68%.
The diff coverage is 86.79%.

@@            Coverage Diff             @@
##             main     #514      +/-   ##
==========================================
+ Coverage   85.89%   88.58%   +2.68%     
==========================================
  Files         143       88      -55     
  Lines        7070     6062    -1008     
  Branches     1855     1632     -223     
==========================================
- Hits         6073     5370     -703     
+ Misses        906      661     -245     
+ Partials       91       31      -60     

Impacted Files	Coverage Δ
packages/appsync-modelgen-plugin/src/index.ts	0.00% <0.00%> (ø)
...elgen-plugin/src/interfaces/introspection/index.ts	0.00% <0.00%> (ø)
...lugin/src/interfaces/introspection/model-schema.ts	0.00% <0.00%> (ø)
packages/appsync-modelgen-plugin/src/plugin.ts	0.00% <0.00%> (ø)
packages/appsync-modelgen-plugin/src/preset.ts	0.00% <0.00%> (ø)
...ges/appsync-modelgen-plugin/src/utils/constants.ts	100.00% <ø> (ø)
...-plugin/src/visitors/appsync-javascript-visitor.ts	97.82% <88.88%> (-2.18%)	⬇️
...-plugin/src/visitors/appsync-typescript-visitor.ts	81.81% <93.33%> (+1.52%)	⬆️
...sync-modelgen-plugin/src/utils/process-has-many.ts	90.36% <96.00%> (+2.43%)	⬆️
...rc/visitors/appsync-model-introspection-visitor.ts	98.70% <98.70%> (ø)
... and 65 more

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