Parser: Introduce HTMLConditionalOpenTagNode

[marcoroth]

This pull request adds a new HTMLConditionalOpenTagNode AST node type for representing ERB conditionals that only vary the open tag (typically attributes) while sharing the same tag name, body content, and close tag.

<% if @condition %>
  <div class="a">
<% else %>
  <div class="b">
<% end %>
  Content
</div>

Previously, this pattern was incorrectly parsed as mismatched tags. Now, Herb detects this pattern and creates a proper HTMLElementNode with an HTMLConditionalOpenTagNode as its open_tag, allowing the formatter, linter, and other tools to understand the structure correctly:

@ DocumentNode (location: (1:0)-(8:0))
└── children: (2 items)
    ├── @ HTMLElementNode (location: (1:0)-(7:6))
    │   ├── open_tag:
    │   │   └── @ HTMLConditionalOpenTagNode (location: (1:0)-(5:9))
    │   │       ├── conditional:
    │   │       │   └── @ ERBIfNode (location: (1:0)-(5:9))
    │   │       │       ├── tag_opening: "<%" (location: (1:0)-(1:2))
    │   │       │       ├── content: " if some_condition " (location: (1:2)-(1:21))
    │   │       │       ├── tag_closing: "%>" (location: (1:21)-(1:23))
    │   │       │       ├── then_keyword: ∅
    │   │       │       ├── statements: (3 items)
    │   │       │       │   ├── @ HTMLTextNode (location: (1:23)-(2:2))
    │   │       │       │   │   └── content: "\n  "
    │   │       │       │   │
    │   │       │       │   ├── @ HTMLOpenTagNode (location: (2:2)-(2:17))
    │   │       │       │   │   ├── tag_opening: "<" (location: (2:2)-(2:3))
    │   │       │       │   │   ├── tag_name: "div" (location: (2:3)-(2:6))
    │   │       │       │   │   ├── tag_closing: ">" (location: (2:16)-(2:17))
    │   │       │       │   │   ├── children: (1 item) [...]
    │   │       │       │   │   └── is_void: false
    │   │       │       │   │
    │   │       │       │   └── @ HTMLTextNode (location: (2:17)-(3:0))
    │   │       │       │       └── content: "\n"
    │   │       │       │
    │   │       │       ├── subsequent:
    │   │       │       │   └── @ ERBElseNode (location: (3:0)-(5:0))
    │   │       │       │       ├── tag_opening: "<%" (location: (3:0)-(3:2))
    │   │       │       │       ├── content: " else " (location: (3:2)-(3:8))
    │   │       │       │       ├── tag_closing: "%>" (location: (3:8)-(3:10))
    │   │       │       │       └── statements: (3 items)
    │   │       │       │           ├── @ HTMLTextNode (location: (3:10)-(4:2))
    │   │       │       │           │   └── content: "\n  "
    │   │       │       │           │
    │   │       │       │           ├── @ HTMLOpenTagNode (location: (4:2)-(4:17))
    │   │       │       │           │   ├── tag_opening: "<" (location: (4:2)-(4:3))
    │   │       │       │           │   ├── tag_name: "div" (location: (4:3)-(4:6))
    │   │       │       │           │   ├── tag_closing: ">" (location: (4:16)-(4:17))
    │   │       │       │           │   ├── children: (1 item) [...]
    │   │       │       │           │   └── is_void: false
    │   │       │       │           │
    │   │       │       │           └── @ HTMLTextNode (location: (4:17)-(5:0))
    │   │       │       │               └── content: "\n"
    │   │       │       │
    │   │       │       └── end_node:
    │   │       │           └── @ ERBEndNode (location: (5:0)-(5:9))
    │   │       │               ├── tag_opening: "<%" (location: (5:0)-(5:2))
    │   │       │               ├── content: " end " (location: (5:2)-(5:7))
    │   │       │               └── tag_closing: "%>" (location: (5:7)-(5:9))
    │   │       │
    │   │       ├── tag_name: "div" (location: (2:3)-(2:6))
    │   │       └── is_void: false
    │   │
    │   ├── tag_name: "div" (location: (2:3)-(2:6))
    │   ├── body: (1 item)
    │   │   └── @ HTMLTextNode (location: (5:9)-(7:0))
    │   │       └── content: "\n  Content\n"
    │   │
    │   ├── close_tag:
    │   │   └── @ HTMLCloseTagNode (location: (7:0)-(7:6))
    │   │       ├── tag_opening: "</" (location: (7:0)-(7:2))
    │   │       ├── tag_name: "div" (location: (7:2)-(7:5))
    │   │       ├── children: []
    │   │       └── tag_closing: ">" (location: (7:5)-(7:6))
    │   │
    │   ├── is_void: false
    │   └── source: "HTML"
    │
    └── @ HTMLTextNode (location: (7:6)-(8:0))
        └── content: "\n"

Follow up on #1146

Resolves #83
Resolves #398
Resolves #490
Resolves #779

[pkg-pr-new]

npx https://pkg.pr.new/@herb-tools/formatter@1153

npx https://pkg.pr.new/@herb-tools/language-server@1153

npx https://pkg.pr.new/@herb-tools/linter@1153

commit: 46a0590

[github-actions]

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