feat($markdown): TOC component (close: #1275)

[shigma]

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

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:

Another Reason for Using TOC Component

TOC component provides an ability to access the current page structure. For example, with TOC component, we can easily support a table of contents that floats on the right of the page.

Potential Breaking Changes and Migrations

slugify

• use markdown.slugify instead.

format

• currently not supported by $page.headers.
• also we cannot transfer function from server to client.
• future solutions: markdown.format: function for formatting headings.

forceFullToc

• currently not supported by $page.headers.
• we may support this option globally in the future.

[ulivz]

Great work!