PR tab, diffing and code highlight update

[deamme]

Fixes #36

There are still some improvements that can be made but let's get this merged for now.

One issue is the diffing component that I'm using - it doesn't diff properly at the last line of a file. An example of this is shown below.

Old code:

const a = 10
const b = 10
const c = () => console.log('foo')

if(a > 10) {
  console.log('bar')
}

console.log('done')

New code:

const a = 10
const boo = 10

if(a === 10) {
  console.log('bar')
}