Use TS strict mode in a11y addon sources

[gaetanmaisse]

What I did

Use TS strict mode in a11y addon sources. But why?

• strict mode is included in the default tsconfig.json generated by tsc CLI
• Stricter rules help to find bugs faster than ever, for instance I found and fix fix(a11y): fix highlight of selected blindness color filter #9179 by using some of the rules I activated in tsconfig.json
• Some of our users are using TS in strict mode and are expecting that the types we are exposing are working in strict mode (e.g. Storybook 5.2 has issues with TypeScript types #8233)
• noUnusedLocals put in light the fact that some variables were not used for at least 8 months but still there and still requiring maintenance.

How to test

• All tests should pass
• As it is refactoring, everything should work like before

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[ndelangen]

How does this relate to #8883?

[gaetanmaisse]

@ndelangen I just take a look at #8883, there will be only minor conflicts because this PR focus on refactoring/cleaning some TS (strict mode) issues and not deeply modifying how the addon works. #8883 can be merged (but not immediately as we must wait for 6.0 I guess, just see that you updated the base branch for next-6.0.0), then just tag me and I will fix the conflicts with this branch. 🎄🎅