Add Ligatures

This merge request adds ligatures for the following combinations: '=>', '->', '==', '!=', '===', '!==', '<=', '>=', ':='

It also adjusts the height of the following characters to better align for ligatures: '+', '-', '='

Preview: https://imgur.com/CTQykH1

The guides in the some of the glyphs may be off, I was unable to fix those. Let me know of any adjustments that may be wanted.