feat(eslint-plugin): [prefer-output-readonly] add suggestion

[rafaelss95]

To be reviewed after #458's merge.

Note that I went with a suggest instead of fix, because if we fix it directly we can break the code since there is no guarantee that @Output property is not being mutated somewhere.

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[JamesHenry]

Having it as a suggestion makes sense, thanks! But I don't see any reason not to make the message key descriptive, it is what the eslint docs show in all their examples: https://eslint.org/docs/developer-guide/working-with-rules#providing-suggestions

...and allows for multiple suggestions to more easily be provided if applicable.

Suggested change

	suggestFix: 'Add readonly modifier',
	addReadonlyModifier: 'Add readonly modifier',

[JamesHenry]

(Note the suggestion was just an easy way to add the feedback, it would need to be applied in all locations across multiple files)

[rafaelss95]

Good idea, I went with suggestAddReadonlyModifier after seeing many cases with the suggest prefix in @typescript-eslint repo as https://github.com/typescript-eslint/typescript-eslint/blob/1c1b572c3000d72cfe665b7afbada0ec415e7855/packages/eslint-plugin/src/rules/no-non-null-assertion.ts#L22.

[JamesHenry]

Thanks a lot!