Switch minimum Node.js target from 6.5 to 10.0

BREAKING CHANGE: Users of older Node.js versions will need to transpile
this library, or update their environment to a supported Node.js
version.

CONTRIBUTING.md

@@ -25,7 +25,7 @@ npm test # just to be sure
 ## Repository Directory & File Structure
 
 - **`browser/`** - Browser-optimised ES build of the library, which should work in modern browsers. Used automatically by e.g. Webpack & Rollup via the `"browser"` value in `package.json`. A corresponding minimal set of the library's required polyfills is available at `playground/src/polyfill.js`
-- **`dist/`** - Node-optimised build of the library, which should work in Node.js 6.0.0 and later without polyfills.
+- **`dist/`** - Node-optimised build of the library, which should work in Node.js 10.0 and later without polyfills.
 - **`docs/`** - Sources for the library's [documentation site](https://eemeli.org/yaml).
 - **`docs-slate/`** - Compiler for the library's [documentation site](https://eemeli.org/yaml). Maintained as a git submodule to allow merges from its upstream source, [Slate](https://github.com/slatedocs/slate). See its [`README`](./docs-slate/README.md) for installation instructions. Note that the build target is the `gh-pages` branch of _this_ repo.
 - **`playground/`** - Source files for a browser-based [playground](https://eemeli.org/yaml-playground/) using this library. Also contains the Selenium browser tests for the library. Maintained as a git submodule to allow for easier publication.

package.json

@@ -94,6 +94,6 @@
     "typescript": "^4.1.3"
   },
   "engines": {
-    "node": ">= 6"
+    "node": ">= 10"
   }
 }

rollup.node-config.js

@@ -16,7 +16,7 @@ export default {
   plugins: [
     babel({
       babelHelpers: 'bundled',
-      presets: [['@babel/env', { modules: false, targets: { node: '6.5' } }]]
+      presets: [['@babel/env', { modules: false, targets: { node: '10.0' } }]]
     })
   ],
   treeshake: { moduleSideEffects: false, propertyReadSideEffects: false }