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/pipeline.test.ts

@@ -58,7 +58,10 @@ describe('createImagePipeline', () => {
 
   it('accepts PNG and WebP as well as JPEG', async () => {
     assert.equal((await pipeline.process(await photo(800, 600, 'png'), 'a.png')).sourceExt, 'png');
-    assert.equal((await pipeline.process(await photo(800, 600, 'webp'), 'a.webp')).sourceExt, 'webp');
+    assert.equal(
+      (await pipeline.process(await photo(800, 600, 'webp'), 'a.webp')).sourceExt,
+      'webp',
+    );
     assert.deepEqual([...ACCEPTED_MIME_TYPES], ['image/jpeg', 'image/png', 'image/webp']);
   });
 
@@ -105,8 +108,7 @@ describe('createImagePipeline', () => {
       .toBuffer();
     await assert.rejects(
       () => pipeline.process(gif, 'anim.gif'),
-      (error: unknown) =>
-        error instanceof UnsupportedImageError && error.message.includes('gif'),
+      (error: unknown) => error instanceof UnsupportedImageError && error.message.includes('gif'),
     );
   });