Parser: Don't treat backslash as escape character in attribute values

[marcoroth]

This pull request updates the parser to treat backslash as a literal character in HTML attribute values instead of an escape character.

The HTML spec does not define backslash as an escape character in attribute values. To include quote characters in attribute values, the spec recommends using HTML character references (e.g. " or ') or using a different quote style.

See: https://html.spec.whatwg.org/multipage/syntax.html#attributes-2

The following snippet:

<button data-shortcut="S,s,\">S \</button>

Now parses as:

@ DocumentNode (location: (1:0)-(1:42))
└── children: (3 items)
    ├── @ HTMLOpenTagNode (location: (1:0)-(1:30))
    │   ├── tag_opening: "<" (location: (1:0)-(1:1))
    │   ├── tag_name: "button" (location: (1:1)-(1:7))
    │   ├── tag_closing: ">" (location: (1:29)-(1:30))
    │   ├── children: (1 item)
    │   │   └── @ HTMLAttributeNode (location: (1:8)-(1:29))
    │   │       ├── name: 
    │   │       │   └── @ HTMLAttributeNameNode (location: (1:8)-(1:21))
    │   │       │       └── children: (1 item)
    │   │       │           └── @ LiteralNode (location: (1:8)-(1:21))
    │   │       │               └── content: "data-shortcut"
    │   │       │               
    │   │       ├── equals: "=" (location: (1:21)-(1:22))
    │   │       └── value: 
    │   │           └── @ HTMLAttributeValueNode (location: (1:22)-(1:29))
    │   │               ├── open_quote: """ (location: (1:22)-(1:23))
    │   │               ├── children: (1 item)
    │   │               │   └── @ LiteralNode (location: (1:23)-(1:28))
    │   │               │       └── content: "S,s,\\"
    │   │               │   
    │   │               ├── close_quote: """ (location: (1:28)-(1:29))
    │   │               └── quoted: true
    │   │               
    │   └── is_void: false
    │   
    ├── @ HTMLTextNode (location: (1:30)-(1:33))
    │   └── content: "S \\"
    │   
    └── @ HTMLCloseTagNode (location: (1:33)-(1:42))
        ├── tag_opening: "</" (location: (1:33)-(1:35))
        ├── tag_name: "button" (location: (1:35)-(1:41))
        └── tag_closing: ">" (location: (1:41)-(1:42))

Resolves #1211

[pkg-pr-new]

npx https://pkg.pr.new/@herb-tools/formatter@1213

npx https://pkg.pr.new/@herb-tools/language-server@1213

npx https://pkg.pr.new/@herb-tools/linter@1213

commit: 6519a74

[github-actions]

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