feat(playground): Implement editable input variable textareas (#4987) (#5006)

* feat(playground): Implement editable input variable textareas (#4987)

* docs(playground): Improve commenting around playground store usage

* refactor(playground): Switch to mostly unstyled codemirror for variable editing

* fix(playground): Fix styling issues and flickering when rendering playground variable inputs

* refactor(playground): Add and use type guards for playground input type

* docs(playground): update comment on variable cache mechanics

* fix(playground): fix types and tests post rebase