feat(theme): support dynamic headers and nesting in outline

[brc-dd]

Most of the nesting-related stuff is copied as is from #965. (Huge thanks to @fi3ework!)

fixes #785, fixes #954, closes #965

---

⚠️ For those who are using/extending the default theme:

This PR changes meaning of outline: deep -- earlier it meant [2, 3], now it means [2, 6]. (Some projects are using it although it was never documented.)

[brc-dd]

Looking at the generated files now, I believe we can significantly reduce the payload size if we disable headers. I mean just provide an option so users can do that (there is no need for them unless someone is doing custom stuff like in vuejs/theme). For backwards compatibility, let's keep it enabled by default. (See below.)

Example:

This is the result of our guide/markdown.md's lean build (~2.4KB):

import{_ as s,o as n,c as a,a as l}from"./chunks/framework.cacadd97.js";const C=JSON.parse('{"title":"Markdown Extensions","description":"","frontmatter":{},"headers":[{"level":2,"title":"Header Anchors","slug":"header-anchors","link":"#header-anchors","children":[]},{"level":2,"title":"Links","slug":"links","link":"#links","children":[{"level":3,"title":"Internal Links","slug":"internal-links","link":"#internal-links","children":[]},{"level":3,"title":"Page Suffix","slug":"page-suffix","link":"#page-suffix","children":[]},{"level":3,"title":"External Links","slug":"external-links","link":"#external-links","children":[]}]},{"level":2,"title":"Frontmatter","slug":"frontmatter","link":"#frontmatter","children":[]},{"level":2,"title":"GitHub-Style Tables","slug":"github-style-tables","link":"#github-style-tables","children":[]},{"level":2,"title":"Emoji \u{1F389}","slug":"emoji","link":"#emoji","children":[]},{"level":2,"title":"Table of Contents","slug":"table-of-contents","link":"#table-of-contents","children":[]},{"level":2,"title":"Custom Containers","slug":"custom-containers","link":"#custom-containers","children":[{"level":3,"title":"Default Title","slug":"default-title","link":"#default-title","children":[]},{"level":3,"title":"Custom Title","slug":"custom-title","link":"#custom-title","children":[]},{"level":3,"title":"raw","slug":"raw","link":"#raw","children":[]}]},{"level":2,"title":"Syntax Highlighting in Code Blocks","slug":"syntax-highlighting-in-code-blocks","link":"#syntax-highlighting-in-code-blocks","children":[]},{"level":2,"title":"Line Highlighting in Code Blocks","slug":"line-highlighting-in-code-blocks","link":"#line-highlighting-in-code-blocks","children":[]},{"level":2,"title":"Line Numbers","slug":"line-numbers","link":"#line-numbers","children":[]},{"level":2,"title":"Import Code Snippets","slug":"import-code-snippets","link":"#import-code-snippets","children":[]},{"level":2,"title":"Markdown File Inclusion","slug":"markdown-file-inclusion","link":"#markdown-file-inclusion","children":[]},{"level":2,"title":"Advanced Configuration","slug":"advanced-configuration","link":"#advanced-configuration","children":[]}],"relativePath":"guide/markdown.md","lastUpdated":1661916041000}'),e={name:"guide/markdown.md"},p=l("",123),o=[p];function t(c,r,i,d,y,D){return n(),a("div",null,o)}const g=s(e,[["render",t]]);export{C as __pageData,g as default};

We only need this (~350B) (also removed empty frontmatter and description, I guess they can be removed if empty while building and empty objects/arrays can be added when we load page data so that the components accessing them don't break):

import{_ as s,o as n,c as a,a as l}from"./chunks/framework.cacadd97.js";const C=JSON.parse('{"title":"Markdown Extensions","relativePath":"guide/markdown.md","lastUpdated":1661916041000}'),e={name:"guide/markdown.md"},p=l("",123),o=[p];function t(c,r,i,d,y,D){return n(),a("div",null,o)}const g=s(e,[["render",t]]);export{C as __pageData,g as default};

---

I just noticed that we can already do this:

  markdown: {
    headers: {
      level: [0, 0]
    }
  },

Done this, got around ~40KB reduction in size.

[LittleSound]

I really need the functionality provided by the outline component fixed in this Pull Request, when will a new version of VitePress be released?