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-emphasis.md

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+# Basic Emphasis Test
+
+This file tests bold and italic text formatting using asterisks and underscores.
+
+## Bold Text
+
+Bold text is created using two asterisks or two underscores before and after the text.
+
+I just love **bold text**.
+
+I also love __bold text with underscores__.
+
+**Bold text** can appear at the start of a sentence.
+
+A sentence can end with **bold text**.
+
+A sentence can have **bold text** in the middle.
+
+## Italic Text
+
+Italic text is created using one asterisk or one underscore before and after the text.
+
+Italicized text is the *cat's meow*.
+
+Italicized text is also the _cat's meow_.
+
+*Italic text* can appear at the start of a sentence.
+
+A sentence can end with *italic text*.
+
+A sentence can have *italic text* in the middle.
+
+## Bold and Italic Together
+
+To emphasize text with both bold and italics, use three asterisks or three underscores.
+
+This text is ***bold and italic***.
+
+This text is also ___bold and italic___.
+
+***Bold and italic*** can appear at the start of a sentence.
+
+A sentence can end with ***bold and italic***.
+
+A sentence can have ***bold and italic*** in the middle.
+
+## Mid-Word Emphasis
+
+You can emphasize the middle of a word for emphasis.
+
+Love**is**bold
+
+Love*is*italic
+
+Love***is***bolditalic
+
+## Best Practices
+
+### Use Asterisks for Mid-Word Emphasis
+
+For compatibility, use asterisks when emphasizing the middle of a word.
+
+Love**is**bold (correct)
+
+Love__is__bold (may not work in all processors)
+
+A*cat*meow (correct)
+
+A_cat_meow (may not work in all processors)
+
+### Spacing Around Emphasis
+
+Emphasis markers should be directly adjacent to the text being emphasized.
+
+This is **correct** spacing.
+
+This is ** incorrect ** spacing.
+
+This is *correct* spacing.
+
+This is * incorrect * spacing.
+
+## Multiple Emphasis in One Paragraph
+
+A single paragraph can contain multiple instances of bold, italic, and combined emphasis.
+
+This paragraph has **bold text**, *italic text*, and ***bold italic text*** all together. You can use **multiple bold** sections and *multiple italic* sections in the same paragraph.
+
+## Emphasis with Punctuation
+
+Emphasis works correctly with adjacent punctuation marks.
+
+**Bold text**, with a comma.
+
+**Bold text.** With a period.
+
+**Bold text!** With an exclamation.
+
+**Bold text?** With a question mark.
+
+*Italic text*, with a comma.
+
+*Italic text.* With a period.
+
+*Italic text!* With an exclamation.
+
+*Italic text?* With a question mark.
+
+## Emphasis at Word Boundaries
+
+Emphasis can appear at the start or end of words.
+
+**Start** of a word.
+
+End of a **word**.
+
+*Start* of a word.
+
+End of a *word*.
+
+## Emphasis with Links
+
+Emphasis can be combined with links.
+
+This is a [**bold link**](https://example.com).
+
+This is a [*italic link*](https://example.com).
+
+This is a [***bold italic link***](https://example.com).
+
+## Emphasis with Code
+
+Emphasis cannot be used inside code blocks, but can appear alongside inline code.
+
+This has `code` and **bold** together.
+
+This has `code` and *italic* together.
+
+## Nested Emphasis
+
+You cannot nest emphasis of the same type, but you can combine different types.
+
+***Bold and italic*** is valid.
+
+**Bold with *italic inside* bold** is valid.
+
+*Italic with **bold inside** italic* is valid.
+
+## Edge Cases
+
+### Emphasis with Only Spaces
+
+**   **
+
+*   *
+
+### Emphasis with Special Characters
+
+**Bold with !@#$%**
+
+*Italic with !@#$%*
+
+### Very Short Emphasis
+
+****
+
+***
+
+**
+
+*
+
+### Emphasis Markers Without Closing
+
+**Bold text without closing
+
+*Italic text without closing
+
+### Emphasis with Numbers
+
+**123**
+
+*456*
+
+### Emphasis with Only Punctuation
+
+**!!!**
+
+*???*
+