feat: add basic markdown syntax test files

Add comprehensive test suite for basic markdown syntax features:
- basic-headings.md: All heading levels and setext syntax
- basic-paragraphs-line-breaks.md: Paragraph separation and line breaks
- basic-emphasis.md: Bold and italic formatting
- basic-blockquotes.md: Blockquotes with nested content
- basic-lists.md: Ordered and unordered lists with nesting
- basic-code.md: Inline code and code blocks
- basic-horizontal-rules.md: Horizontal rule variants
- basic-links-and-images.md: Links and images with various syntax
- basic-escaping.md: Character escaping
- basic-index.md: Index of all test files

All files follow the Markdown Guide's Basic Syntax specification.

test/markdown/basic-headings.md

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+# Basic Headings Test
+
+This file tests markdown heading syntax, including all heading levels and alternate syntax forms.
+
+## Heading Levels
+
+Headings are created using number signs (`#`) followed by a space and the heading text. The number of `#` symbols determines the heading level.
+
+# Heading Level 1
+
+## Heading Level 2
+
+### Heading Level 3
+
+#### Heading Level 4
+
+##### Heading Level 5
+
+###### Heading Level 6
+
+## Best Practices
+
+Always include a space between the number signs and the heading text for compatibility across markdown processors.
+
+# Correct: Space after #
+
+#Incorrect: No space after #
+
+## Blank Lines
+
+For best compatibility, include blank lines before and after headings.
+
+This paragraph is before the heading.
+
+# Heading with blank lines
+
+This paragraph is after the heading.
+
+Without blank lines, this might not render correctly.
+# Heading without blank lines
+This text might be treated as part of the heading.
+
+## Alternate Syntax (Setext)
+
+Heading level 1 can also be created using equals signs (`=`) on the line below the text.
+
+Heading Level 1
+===============
+
+Heading level 2 can be created using hyphens (`-`) on the line below the text.
+
+Heading Level 2
+---------------
+
+## Headings with Formatting
+
+Headings can contain inline formatting like bold and italic text.
+
+### Heading with **Bold** Text
+
+### Heading with *Italic* Text
+
+### Heading with ***Bold and Italic*** Text
+
+### Heading with `Code` Text
+
+## Headings with Links
+
+Headings can contain links.
+
+### Heading with [Link](https://example.com)
+
+### Heading with [Reference Link][ref]
+
+[ref]: https://example.com
+
+## Long Headings
+
+This tests how headings handle very long text that might wrap across multiple lines on smaller screens or in narrow containers.
+
+# This is a very long heading that contains many words and should demonstrate how the markdown processor handles headings that extend beyond a single line of text
+
+## Special Characters in Headings
+
+Headings can contain various special characters and punctuation.
+
+### Heading with Numbers: 123
+
+### Heading with Symbols: !@#$%^&*()
+
+### Heading with Quotes: "Hello World"
+
+### Heading with Parentheses (Like This)
+
+### Heading with Brackets [Like This]
+
+### Heading with Braces {Like This}
+
+## Multiple Headings
+
+Multiple headings of the same or different levels can appear consecutively.
+
+# First H1
+
+# Second H1
+
+## First H2
+
+## Second H2
+
+### First H3
+
+### Second H3
+
+## Edge Cases
+
+### Heading with Only Spaces
+
+#    
+
+### Heading with Trailing Spaces
+
+# Heading with trailing spaces    
+
+### Heading Starting with Number Sign
+
+# #Heading that starts with a number sign
+
+### Very Short Heading
+
+# A
+
+### Heading with Only Special Characters
+
+# !@#$%^&*()
+