feat: Extensions

[Dhravya]

Extensions will be a set of pre-defined cakes.

Basically, this will allow users to "stack" cakes, smaller cakefiles

Extensions will be a list of other published cakes or maybe even local cakes.

[metadata]
name = "..."
description = "..."
exts = ["readme","license"]

these exts will be run before the actual cake. So you don't have to write extra code. and ask the questions.