Parser: Parse CSS and <style> tags using Lightning CSS

[marcoroth]

This pull request integrates the Rust lightningcss crate for parsing CSS.

Additionally, it updates the Herb Parser to parse the CSS content of <style> tags within HTML+ERB documents using the herb_css_parse function.

So a document like this:

<style>
  body {
    color: black;
    font-size: 14pt;
    margin: 6px;
  }
</style>

Gets parsed as:

@ DocumentNode (location: (1:0)-(8:0))
└── children: (2 items)
    ├── @ HTMLElementNode (location: (1:0)-(7:8))
    │   ├── open_tag:
    │   │   └── @ HTMLOpenTagNode (location: (1:0)-(1:7))
    │   │       ├── tag_opening: "<" (location: (1:0)-(1:1))
    │   │       ├── tag_name: "style" (location: (1:1)-(1:6))
    │   │       ├── tag_closing: ">" (location: (1:6)-(1:7))
    │   │       ├── children: []
    │   │       └── is_void: false
    │   │
    │   ├── tag_name: "style" (location: (1:1)-(1:6))
    │   ├── body: (1 item)
    │   │   └── @ CSSStyleNode (location: (1:7)-(7:0))
    │   │       ├── content: "\n  body {\n    text: black;\n    font-size: 14pt;\n    margin: 6px;\n  }\n"
    │   │       ├── rules: (1 item)
    │   │       │   └── @ CSSRuleNode (location: (1:7)-(7:0))
    │   │       │       ├── selector: "SelectorList([Selector(body, specificity = 0x1)])"
    │   │       │       └── declarations: (3 items)
    │   │       │           ├── @ CSSDeclarationNode (location: (1:7)-(7:0))
    │   │       │           │   ├── property: "color"
    │   │       │           │   └── value: "Color(RGBA(RGBA { red: 0, green: 0, blue: 0, alpha: 255 }))"
    │   │       │           │
    │   │       │           ├── @ CSSDeclarationNode (location: (1:7)-(7:0))
    │   │       │           │   ├── property: "font-size"
    │   │       │           │   └── value: "FontSize(Length(Dimension(Pt(14.0))))"
    │   │       │           │
    │   │       │           └── @ CSSDeclarationNode (location: (1:7)-(7:0))
    │   │       │               ├── property: "margin"
    │   │       │               └── value: "Margin(Margin { top: LengthPercentage(Dimension(Px(6.0))), right: LengthPercentage(Dimension(Px(6.0))), bottom: LengthPercentage(Dimension(Px(6.0))), left: LengthPercentage(Dimension(Px(6.0))) })"
    │   │       │
    │   │       │
    │   │       ├── valid: true
    │   │       └── parse_error: ""
    │   │
    │   ├── close_tag:
    │   │   └── @ HTMLCloseTagNode (location: (7:0)-(7:8))
    │   │       ├── tag_opening: "</" (location: (7:0)-(7:2))
    │   │       ├── tag_name: "style" (location: (7:2)-(7:7))
    │   │       ├── children: []
    │   │       └── tag_closing: ">" (location: (7:7)-(7:8))
    │   │
    │   ├── is_void: false
    │   └── source: "HTML"
    │
    └── @ HTMLTextNode (location: (7:8)-(8:0))
        └── content: "\n"