feat(types): Add ModelDefined type as syntactic sugar

[Keimeno]

Pull Request check-list

Please make sure to review and check all of these items:

• Does npm run test or npm run test-DIALECT pass with this change (including linting)?
• Does the description below contain a link to an existing issue (Closes #[issue]) or a description of the issue you are solving?
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• Did you update the typescript typings accordingly (if applicable)?
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Description of change

The current way to define a module in a functional way with Typescript works like this:

const Note: ModelCtor<
  Model<
    NoteAttributes, 
    NoteCreationAttributes
  >
> = sequelize.define(
  ...
);

However this approach is a bit much to write, and with the ModelDefined type it could be reduced to:

const Note: ModelDefined<
  NoteAttributes, 
  NoteCreationAttributes
> = sequelize.define(
  ...
);

It's just added syntactic sugar, that simplifies things.
Also there is no reference to ModelCtor nor a functional way with Typescript in the docs.

So another change is, I've added an example for defining models in a functional way in Typescript.

[codecov]

Codecov Report

Merging #12445 into v6 will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##               v6   #12445      +/-   ##
==========================================
- Coverage   96.44%   96.42%   -0.03%     
==========================================
  Files          95       95              
  Lines        9154     9154              
==========================================
- Hits         8829     8827       -2     
- Misses        325      327       +2     

Impacted Files	Coverage Δ
lib/dialects/postgres/connection-manager.js	94.40% <0.00%> (-1.40%)	⬇️

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[sushantdhiman]

NOTE: Keep the following code in sync with typescriptDocs/ModelInit.ts to ensure it typechecks correctly.

Please take care of that

[Keimeno]

Is synchronized now. Also commented out the Notice about synchronization

[sushantdhiman]

🎉 This PR is included in version 6.3.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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