feat: plugin clean urls

[shigma]

Summary

Add a new plugin @vuepress/vuepress-plugin-clean-urls.

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 ([Feature request] Cleaner and friendlier URLs #608 )
• 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:

This plugin supports two options:

normalSuffix

• Type: string
• Default: ''

The suffix for normal pages. For example, foo/bar.md will become:

• foo/bar.html by default (without this plugin)
• foo/bar/ (with normalSuffix set to '/')
• foo/bar (with normalSuffix set to '')

indexSuffix

• Type: string
• Default: '/'

The suffix for index pages. For example, foo/index.md will become:

• foo/ by default (without this plugin)
• foo (with indexSuffix set to '')
• foo/index.html (with indexSuffix set to '/index.html')