MDN React Tutorials mdn#4: Hooks and state (mdn#30977)

* chore: clean up intro

* chore: .js

* chore: fix import statements

* chore: small nit

* chore: disambiguate "e" as "event"

* chore: add "at this point" checks

* chore: fix final app.jsx check

* small tweak

* Small tweak

* Small tweak

* chore: suggestions from code review

Co-authored-by: Chris Mills <[email protected]>

* chore: clarify form submission section

* chore: add naming convention note

* chore: rewrite state section

* chore: nits

* Small tweak

* Small tweaks to new section

* Small tweaks to updated state section

* chore: apply suggestions from code review

Co-authored-by: Chris Mills <[email protected]>

* chore: clarify ecosystem statement

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Co-authored-by: Chris Mills <[email protected]>