chore: require readonly properties to be immutable

[galargh]

• Because this PR includes a bug fix, relevant tests have been included.
• Because this PR includes a new feature, the change was previously discussed on an Issue or with someone from the team.
• I didn't do anything of this.

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This draft showcases an example implementation of #6005.

It enforces readonly props immutability via an ESLint plugin - https://github.com/galargh/eslint-plugin-immutable-readonly

It needs further discussion whether that's something that we want to enforce automatically.

It also needs further work as, currently, the plugin is only published to the GitHub packages (downloading from GitHub packages requires authentication with a GitHub token).

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[galargh]

I'm going to close this draft for now, I just wanted to be able to link to an example implementation from the issue that captures this work item - #6005