feat: adds support for loading external theme CSS for MFEs

Introduces `useAppTheme` in `AppProvider` to load/inject `<link>` elements for the core theme CSS and any theme variant CSS into the HTML document. Exposes the app theme state and a way to mutate it to consumers via `AppContext`.

fix: handle missing theme config

fix: add env vars to env files

fix: remove unused code

chore: wip commit

fix: prefer runtime config for paragon theme

fix: rebase on master and resolve package conflicts

fix: revert to original webpack.dev.config config

chore: clean up unnecessary comment line

fix: grammar

Co-authored-by: Adolfo R. Brandes <[email protected]>

chore: refresh package-lock.json to lockfileVersion 3

fix: clean up link nodes in document head when no longer needed

fix: fallback to locally installed core and light theme css urls

docs: update to docs

chore: update docs about

fix: expose setThemeVariant

fix: rebase with master and update based on PARAGON changes

chore: remove support for env vars config for paragon dynamic theming

fix: updates

fix: one more update

fix: refresh package-lock.json

fix: refresh package-lock.json pt2

fix: updates

fix: update package-lock.json

fix: make it theme variant agnostic

docs: update howto theming guide

fix: ensure app loads without styles if the PARAGON_THEME_URLS and fallback urls all fail to load

fix: ensure fallback theme links are removed if they also error

docs: add link to mfe runtime config api adr

fix: don't attempt to load paragon css urls if PARAGON_THEME_URLS is absent

docs: fix code example

docs: add missing comma

docs: update how to

chore: update package-lock.json

chore: update package-lock.json take 2

chore: remove console.log statements

fix: ignore system preference change when theme variant set in localstorage

chore: add tests for updates to AppProvider

chore: update react-intl to pass peer dependencies after pinning all deps

chore: split hooks.js up into separate files and begin some related tests

test: add testing to useParagonTheme hooks (openedx#514)

* test: add testing to useParagonThemeCore
* test: add test to useThemeVariants hook
* fix: Paragon definition and remove onload mock
* test: change test message to be clear