Feature request: Make block spacing configurable via MarkdownRenderConfig #87

Problem

All block-level spacing in the library is currently hardcoded in BlockView.swift:

• Inter-block spacing: VStack(alignment: .leading, spacing: 30) — the gap between paragraphs, headings, code blocks, etc.
• Paragraph line spacing: ParagraphView(contents: contents, lineSpacing: 5) — additional line spacing within paragraph blocks

MarkdownRenderConfig already provides a clean configuration surface for fonts and colors, but has no spacing properties. For apps that want to adjust the visual density of rendered Markdown (e.g., a chat bubble needs tighter spacing than a full-width article view), there is currently no way to do this without forking.

Proposed solution

Add a blockSpacing property to MarkdownRenderConfig that controls the inter-block gap, preserving the current value of 30 as the default:

Changes to MarkdownRenderConfig

/// Vertical spacing between adjacent blocks (paragraphs, headings,
/// code blocks, lists, etc.). Defaults to 30.
public let blockSpacing: CGFloat

Add it to the init, the static let default, and provide a withBlockSpacing(value:) builder — matching the existing pattern of every other config property.

Changes to BlockView

Read blockSpacing from @Environment(\.markdownConfig) and pass it into the VStack:

VStack(alignment: .leading, spacing: config.blockSpacing) {

Out of scope (for this PR)

• lineSpacing for paragraphs — that could be a separate discussion
• Per block-type spacing (e.g., tighter after headings) — could follow later if there's demand

Why this fits the library

• Follows the exact same pattern as every existing MarkdownRenderConfig property
• No API breakage — the default is 30, identical to current behavior
• Minimal diff (roughly 4 files touched: MarkdownRenderConfig.swift, BlockView.swift, the builders extension, and possibly DocumentView.swift)
• Enables real use cases without adding complexity

Happy to open a PR if the direction looks good — I've already prototyped this in a fork and confirmed it works end-to-end.