feat: add browser-only WASM build variant

[rxliuli]

Summary

When bundling sql.js for browser environments, tools like Vite, Webpack, and esbuild emit warnings about require("node:fs") and require("node:crypto") being externalized for browser compatibility. These require() calls exist in the emcc-generated code because the default ENVIRONMENT includes node.

This PR adds a new sql-wasm-browser.js build variant that eliminates these warnings:

• New EMFLAGS_WASM_BROWSER in Makefile: same as EMFLAGS_WASM but with -s ENVIRONMENT=web,worker, which tells Emscripten to strip all Node.js code paths at compile time
• New build targets: dist/sql-wasm-browser.js (optimized) and dist/sql-wasm-browser-debug.js (debug)
• Conditional exports in package.json: bundlers targeting browsers automatically resolve to the clean sql-wasm-browser.js via the browser condition, while Node.js and other environments continue using the original sql-wasm.js

Before (bundler output)

[plugin rolldown:vite-resolve] Module "fs" has been externalized for browser compatibility...
[plugin rolldown:vite-resolve] Module "crypto" has been externalized for browser compatibility...
[plugin rolldown:vite-resolve] Module "path" has been externalized for browser compatibility...

After

No sql.js related warnings.

Build comparison

File	require() calls	Size
sql-wasm.js (unchanged)	require("node:fs"), require("node:crypto")	45.7 KB
sql-wasm-browser.js (new)	None	44.3 KB
.wasm binary	Both identical	658 KB

No breaking changes — the original sql-wasm.js is fully preserved as the default export.

Closes #620

Test plan

• Build sql-wasm-browser.js with make dist/sql-wasm-browser.js — succeeds
• Verify sql-wasm-browser.js contains zero require() calls
• Verify sql-wasm.js still contains Node.js support code
• Test in a real Vite/WXT project — bundler warnings eliminated

🤖 Generated with Claude Code

[rxliuli]

Verified with pnpm patch

I've verified this change works in a real project (a Chrome extension built with WXT + Vite 7 + rolldown). After applying the patch:

• All Module "fs"/"crypto"/"path" has been externalized for browser compatibility warnings are completely eliminated
• The inject.js bundle size decreased by ~4KB (924.98 KB → 920.62 KB) due to removal of dead Node.js code paths
• No functional regressions

For pnpm users who need this fix now

You can use pnpm patch to apply this change before it's merged and released. I've attached the patch file — to use it:

1. Download the attached sql.js@1.13.0.patch file to your project's patches/ directory
2. Add to your package.json:

   {
     "pnpm": {
       "patchedDependencies": {
         "sql.js@1.13.0": "patches/sql.js@1.13.0.patch"
       }
     }
   }

3. Run pnpm install

Note: The patch file is large (~766KB) because it includes the pre-built sql-wasm-browser.js and sql-wasm-browser.wasm files, since building from source requires Emscripten.

[rxliuli]

Patch file: https://gist.github.com/rxliuli/06a26d53fa45359c8079834aa20d4369

Download the raw file and place it at patches/sql.js@1.13.0.patch in your project.