Improve tree shakability (#612)

* Add `sideEffects: false` to `libesm/package.json`

* Make tweaks I proposed in convo at #612

* Fix build for Unix (and break it again for Windows in the process)

* Use `node --eval`

* --target es6 for ESM build

@fisker points out at #612 (comment) that ESM modules aren't usable by ES5 environments anyway, so targeting ES5 is pointless

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Co-authored-by: Mark Amery <markrobertamery@gmail.com>

Author: fisker

package.json

@@ -71,8 +71,8 @@
     "clean": "rm -rf libcsm/ libesm/ dist/ coverage/ .nyc_output/",
     "lint": "yarn eslint",
     "build": "yarn lint && yarn generate-esm && yarn generate-cjs && yarn check-types && yarn run-rollup && yarn run-uglify",
-    "generate-cjs": "yarn tsc --module commonjs --outDir libcjs && echo '{\"type\": \"commonjs\"}' > libcjs/package.json",
-    "generate-esm": "yarn tsc --module nodenext --outDir libesm && echo '{\"type\": \"module\"}' > libesm/package.json",
+    "generate-cjs": "yarn tsc --module commonjs --outDir libcjs && node --eval \"fs.writeFileSync('libcjs/package.json', JSON.stringify({type:'commonjs',sideEffects:false}))\"",
+    "generate-esm": "yarn tsc --module nodenext --outDir libesm --target es6 && node --eval \"fs.writeFileSync('libesm/package.json', JSON.stringify({type:'module',sideEffects:false}))\"",
     "check-types": "yarn run-tsd && yarn run-attw",
     "test": "nyc yarn _test",
     "_test": "yarn build && cross-env NODE_ENV=test yarn run-mocha",