chore: script for bumping moc (#278)

## usage: `bash script/bump-moc.sh 0.13.6`

This is very basic for now, we can get fancier as we use it, e.g.
- at start: `git switch main && git pull`
- `git switch -c bump/moc-$MOC_VERSION`
- git commit -m 'chore: bump `moc` to v'$MOC_VERSION
- push it: `git push -u origin bump/moc-$MOC_VERSION`
- make the PR: `open https://github.com/dfinity/motoko-playground/pull/new/bump/moc-$MOC_VERSION`

script/bump-moc.sh

@@ -0,0 +1,15 @@
+#!/usr/bin/env bash
+
+MOC_VERSION=$1
+
+echo upgrading '`moc`' to $MOC_VERSION
+echo
+
+perl -pi -e "s/MOC_VERSION = \"[0-9\.]+\"/MOC_VERSION = \"$MOC_VERSION\"/g" src/App.tsx
+
+curl -L https://github.com/dfinity/motoko/releases/download/${MOC_VERSION}/moc-${MOC_VERSION}.js \
+  > public/moc.js
+
+git add src/App.tsx public/moc.js
+git status -uno
+git diff --staged src/App.tsx