feat: Quickstart and client for Plain JS Script Tags

[clockwork-tien]

Description of Changes

The PR implements the following updates:

• Create a quickstart guide and client support for using SpacetimeDB directly in browser with script tags
• A template to easily work with/test the implementation and get started with script tags

The bundle is served via https://unpkg.com/spacetimedb@latest/dist/browser.bundle.js, which only works after the package is published to npm, therefore to test locally we can use the browser-ts template and local bundle:

1. Build local bundle

cd crates/bindings-typescript
pnpm install
pnpm build

2. Use local bundle built in templateindex.html (swap the URL from unpkg)

- <script src="https://unpkg.com/spacetimedb@latest/dist/browser.bundle.js"></script>
+ <script src="../../crates/bindings-typescript/dist/browser.bundle.js"></script>

3. Open browser-ts/index.html directly in browser along with spacetime start + publish the module spacetime publish --project-path spacetimedb browser-ts

Screenshots

API and ABI breaking changes

Expected complexity level and risk

Testing

[clockwork-tien]

@cloutiertyler Thank you, I have applied the changes from #4167, merged/resolved conflicts and did a minor template description update. The PR is ready for review.

[cloutiertyler]

Removed the SDK-level browser bundle (src/browser/index.ts, the IIFE and ESM browser configs in tsup.config.ts) since nothing in the template actually uses them. The template bundles its own bindings via Vite instead. The existing dist/index.browser.mjs (built from src/index.ts) already covers the browser ESM use case.

Long-term, the plan is to have the code generator emit .js + .d.ts files instead of .ts, which would allow the template to drop the Vite build step entirely and use import maps to resolve spacetimedb imports natively in the browser. At that point, the generated bindings would just work with a <script type="importmap"> pointing spacetimedb at dist/index.browser.mjs on unpkg.