feat: definition list block type

[oobagi]

Problem

There's no way to create definition lists (term + definition pairs). These are useful for glossaries, reference notes, and documentation.

Goal

Add a DefinitionList block type that renders term/definition pairs with clear visual hierarchy.

Markdown format

Term
: Definition text here

Another term
: Its definition

Tasks

• Add DefinitionList to BlockType enum
• Parse definition list syntax in parse.go — detect term\n: definition pattern
• Serialize back to definition list markdown
• Round-trip tests
• Add to / command palette
• Render with bold term and indented definition
• Add styling to theme system
• View mode rendering

Scope

Type: enhancement
Size: medium