fix(build): reduce npm package size by 30% by removing source in source maps

Given that the original source is present in the package, it can still be loaded
as-needed from the filesystem, so this was pure duplication. For a pure web build
without a bundler (who is still doing that?), if source maps are still needed,
this will possibly cause an issue. In that case we can look into committing a different
build file for umd.

Author: STRML

.gitignore

@@ -12,3 +12,4 @@ examples/*.js
 !examples/generate.js
 !examples/vars.js
 .vscode
+*.tgz

dist/react-grid-layout.min.js

@@ -17,7 +17,7 @@ module.exports = {
     libraryTarget: "umd",
     library: "ReactGridLayout"
   },
-  devtool: "source-map",
+  devtool: "nosources-source-map",
   externals: {
     react: {
       commonjs: "react",