feat: Add macro support

New: macros.ts with MacroDef, parseBlockMacro, matchInlineMacro,
buildMacroTags, processInlineMacros.

Macro syntax:
  @user                     — bare, no args
  @user()                   — empty parens, same as bare
  @npc(Goblin King)         — self-closing with args
  @style(box center         — block: no closing paren on first line
  Content here.             — content on subsequent lines
  )                         — closing paren on its own line

Unknown macro names now render as an error:
  <span class="ribbit-error">Unknown macro: @bogus</span>

The verbatim keyword causes the contents to render as literals and also
preserves line breaks.

src/ts/ribbit.ts

@@ -6,6 +6,7 @@ import { HopDown } from './hopdown';
 import { defaultTheme } from './default-theme';
 import { ThemeManager } from './theme-manager';
 import { RibbitEmitter, type RibbitEventMap } from './events';
+import { buildMacroTags, type MacroDef } from './macros';
 import type { RibbitTheme } from './types';
 
 export interface RibbitSettings {
@@ -15,6 +16,7 @@ export interface RibbitSettings {
     currentTheme?: string;
     themes?: RibbitTheme[];
     themesPath?: string;
+    macros?: MacroDef[];
     on?: Partial<RibbitEventMap>;
 }
 
@@ -71,12 +73,14 @@ export class Ribbit {
     converter: HopDown;
     themesPath: string;
     private emitter: RibbitEmitter;
+    private macros: MacroDef[];
 
     constructor(settings: RibbitSettings) {
         this.api = settings.api || null;
         this.element = document.getElementById(settings.editorId || 'ribbit')!;
         this.themesPath = settings.themesPath || './themes';
         this.emitter = new RibbitEmitter();
+        this.macros = settings.macros || [];
         this.states = {
             VIEW: 'view',
         };
@@ -89,8 +93,8 @@ export class Ribbit {
         this.themes = new ThemeManager(defaultTheme, this.themesPath, (theme, previous) => {
             this.theme = theme;
             this.converter = theme.tags
-                ? new HopDown({ tags: theme.tags })
-                : new HopDown();
+                ? new HopDown({ tags: theme.tags, macros: this.macros })
+                : new HopDown({ macros: this.macros });
             this.cachedHTML = null;
             this.emitter.emit('themeChange', {
                 current: theme,
@@ -110,8 +114,8 @@ export class Ribbit {
         this.themes.set(activeName);
         this.theme = this.themes.current();
         this.converter = this.theme.tags
-            ? new HopDown({ tags: this.theme.tags })
-            : new HopDown();
+            ? new HopDown({ tags: this.theme.tags, macros: this.macros })
+            : new HopDown({ macros: this.macros });
 
         (settings.plugins || []).forEach(plugin => {
             this.enabledPlugins[plugin.name] = new plugin({