build: modernize makem.js for Windows and current emscripten

- Rewrite tools/makem.js: switch from exec(string) to execFile(array),
  expand globs in Node, locate emcc.py and invoke via python (sidesteps
  Node's CVE-2024-27980 .bat block and cmd.exe's 8k command-line limit)
- Auto-detect emsdk via EMSDK env var when EMSCRIPTEN is unset
- Use .o instead of deprecated .bc for libar intermediate output
- Drop obsolete --memory-init-file 0 flag (asm.js leftover)
- Drop --js-library jslibrary.js (file removed in #18)
- Replace USE_ES6_IMPORT_META=0 with ENVIRONMENT=web (browser-only)
- Make package.json scripts cross-platform (drop ; chain and `date`)
- emcc emsdk  3.1.60

Refs #27
Refs #31

Author: kalwalt

.gitignore

@@ -1,3 +1,4 @@
 node_modules
 build/libar.bc
+build/libar.o
 doc/*