Add ESLint and Prettier

Prettier owns formatting: .prettierrc.json, format and format:check scripts, and
a one-off pass over the repo so the check is green from the start. Embedded
language formatting is off deliberately - the renderers build HTML in html
tagged templates and letting Prettier reflow those would change the markup
customers receive. docs/ is excluded because it is hand-wrapped prose, and shell
scripts have no Prettier parser.

ESLint covers the JavaScript: the two browser scripts, the build and check
scripts, and its own config. no-var is scoped out for the browser scripts on
purpose; they are plain ES5-shaped IIFEs and a hoisting difference would only
show up in a browser.

ESLint cannot lint the TypeScript here: typescript@7 is the native port with no
JavaScript compiler API, and typescript-eslint requires a 4.8-6.1 peer, so it
refuses to install and forcing it through would leave a parser that cannot read
the compiler. tsc --noEmit stays the TypeScript gate; README records the two
ways to change that.

lint and format:check now run in deploy.sh, and VS Code formats on save with
Prettier and surfaces ESLint findings.

test/validate.test.ts

@@ -18,7 +18,10 @@ afterEach(async () => {
 });
 
 function validate() {
-  return validateSite({ dataDir, sections: { has: (slug) => (SECTIONS as readonly string[]).includes(slug) } });
+  return validateSite({
+    dataDir,
+    sections: { has: (slug) => (SECTIONS as readonly string[]).includes(slug) },
+  });
 }
 
 describe('validateSite', () => {
@@ -62,9 +65,7 @@ describe('validateSite', () => {
 
     const problems = await validate();
     assert.equal(hasFatal(problems), true);
-    assert.ok(
-      problems.some((problem) => problem.message.includes('does not match its directory')),
-    );
+    assert.ok(problems.some((problem) => problem.message.includes('does not match its directory')));
   });
 
   it('fails on a manifest that cannot be parsed', async () => {