introduce tags, structs, and reticles

this was meant to be split into smaller changes, but I realised I edited my existing revision too late.

static/brush-editor.js

@@ -1,12 +1,12 @@
-import { CodeEditor, getLineStart } from "rkgk/code-editor.js";
-import { BrushPreview } from "rkgk/brush-preview.js";
+import { CodeEditor } from "rkgk/code-editor.js";
 
 const defaultBrush = `
 -- This is your brush.
--- Try playing around with the numbers,
+-- Try playing around with the numbers, 
 -- and see what happens!
 
-stroke 8 #000 (vec 0 0)
+withDotter \d ->
+  stroke 8 #000 (d To)
 `.trim();
 
 export class BrushEditor extends HTMLElement {