styled-react: Add ThemeProvider and BaseStyles

[siddharthkp]

• Feature flag: primer_react_use_styled_react_theming
• Towards https://github.com/github/primer/issues/5427
• Copy over ThemeProvider and BaseStyles with some minor changes
  • styled-react/BaseStyles uses createGlobalStyle instead of css modules

We are in the process of removing styled-components from @primer/react and moving styled-components based styling to @primer/styled-react. This includes ThemeProvider, BaseStyles and useTheme

It's important that ThemeProvider and a component or utility (like useTheme) that depends on the theme uses the same instance of Context from styled-components.

To roll this change out safely, we are using a feature flag that can serve as a kill switch to go back to using theme components from @primer/react like today.

To do this, we are

1. re-exporting @primer/react theme components (like ThemeProvider) from @primer/styled-react
2. creating a copy of theme components in @primer/styled-react
3. using the feature flag to switch between re-exported and local theme components

The plan is to roll this change out slowly over a week and then removing the feature flag

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; if selected, include a brief description as to why

Testing & Reviewing

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge
• (GitHub staff only) Integration tests pass at github/github (Learn more about how to run integration tests)

[changeset-bot]

🦋 Changeset detected

Latest commit: 0f30134

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@primer/react	Patch
@primer/styled-react	Minor

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[github-actions]

👋 Hi, this pull request contains changes to the source code that github/github depends on. If you are GitHub staff, we recommend testing these changes with github/github using the integration workflow. Thanks!

[primer-integration]

👋 Hi from github/github-ui! Your integration PR is ready: https://github.com/github/github-ui/pull/4296

[primer-integration]

🟢 ci completed with status success.

[github-actions]

👋 Hi, there are new commits since the last successful integration test. We recommend running the integration workflow once more, unless you are sure the new changes do not affect github/github. Thanks!