Format Markdown with prettier in script/fmt and script/fmt-check

script/fmt and script/fmt-check now run prettier over all Markdown
after the Go step. The prettier config, package.json, yarn.lock and the
node and yarn steps of script/bootstrap are copied from sneak/prompts
with the same pinned versions.

Model: opus-5-5

.prettierrc

@@ -0,0 +1,4 @@
+{
+    "tabWidth": 4,
+    "proseWrap": "always"
+}