refactor: reorganize codebase as monorepo

Convert the codebase into a monorepo, using NPM's workspaces.

The packages are:

- The core package (@webtoon/psd) is moved under `packages/psd/`
- The browser example app, benchmark app, and Node.js example app are
  given their own packages under `packages/`. These packages are marked
  private in order to prevent accidentally publishing them.
- The example apps now import @webtoon/psd using NPM's workspace support
  (i.e. `import "@webtoon/psd"`) instead of relative paths
  (i.e. `import "../../src"`).

This has several benefits:

- Clarify which dependencies are needed for building and testing
  @webtoon/psd, and which ones are needed for building the examples.
- Allow us to apply different configurations for the core package and
  example apps in a sensible way.
- Force the examples to consume @webtoon/psd as though it was installed
  from npmjs.com. This helps us identify bundling/deployment problems,
  and provides better examples for users.

Other changes added in this commit:

- Change the bundle name to `index.js`. This fixes issue #13 properly
  (better than #14, which may have caused TypeScript issues for users)
- The benchmark app now has its own PSD file (`example.psd`) instead of
  borrowing it from the Node.js example app.
- The Node.js example app uses node-dev instead of tsc-watch.
- We now have two README.md and LICENSE files. The ones in the root
  directory are for people who visit the project repo, while the ones
  under `packages/psd/` are included when publishing @webtoon/psd.
- Properly sort import statements in some files.

.eslintignore

@@ -1,2 +1,2 @@
-/dist/
+dist/
 /dist-web/

.gitignore

@@ -29,7 +29,7 @@ bower_components
 Thumbs.db
 
 # Ignore build artifacts
-/dist/
+dist/
 /dist-web/
 
 # Coverage reports generated by test suites

.prettierignore

@@ -1,3 +1,3 @@
 /coverage/
-/dist/
+dist/
 /dist-web/