feat($core): functional siteConfig.evergreen

[shigma]

Summary

siteConfig.evergreen has no access to many properties of context:

// config.js
module.exports = ({ isProd }) => ({
  evergreen: !isProd
  // always `true`, because `context.isProd` is
  // `undefined` when `config.js` is applied
})

Now we can use the following config:

// config.js
module.exports = (context) => ({
  evergreen: () => !context.isProd
  // dev: `true`
  // build: `false`
})

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome
• Firefox
• Safari
• Edge
• IE

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[ulivz]

LGTM