fix 1185, feat: support async function exported in vuepress config

[MartinMuzatko]

Summary

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
• Not relevant, server-side change only

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

• A convincing reason for adding this feature (see issue Support asynchronous config for generated sidebar #1185)
• Related documents have been updated (the docs don't even note that you can export a function, see https://vuepress.vuejs.org/guide/basic-config.html#config-file)
• Related tests have been updated (test continue to work, since process is already async function)

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:

Since process was already an async function, and process is the only function which uses resolveConfigAndInitialize, which is the only function to use loadConfig, this code doesn't introduce many changes, but allows to have async vuepress config files.

[vue-bot]

Hey @MartinMuzatko, thank you for your time and effort spent on this PR, contributions like yours help make Vue better for everyone. Cheers! 💚