pack should support `pack extension new`

[natalieparellano]

Description

This is a sub-task of #1470

Just like scaffold new buildpacks, we should be able to scaffold new extensions

Proposed solution

The command should work just like pack buildpack new, but with extensions:

Usage:
  pack buildpack new <id> [flags]

Examples:
pack buildpack new sample/my-buildpack

Flags:
  -a, --api string       Buildpack API compatibility of the generated buildpack (default "0.8")
  -h, --help             Help for 'new'
  -p, --path string      Path to generate the buildpack
  -s, --stacks strings   Stack(s) this buildpack will be compatible with
                         Repeat for each stack in order, or supply once by comma-separated list (default [io.buildpacks.stacks.bionic])
  -V, --version string   Version of the generated buildpack (default "1.0.0")

Additional context

• This feature should be documented somewhere

[natalieparellano]

@AidanDelaney should we aim to make use of scafall? (in addition or instead of?)

[AidanDelaney]

I'd love to see scafall used here.

[natalieparellano]

Let's block this on #1534 since whatever we do for buildpacks can be modified for extensions

[harikannan512]

@natalieparellano I'd like to take a swing at this when unblocked. Assign me this issue, please.

[natalieparellano]

Still blocked on #1534

@harikannan512 I apologize for completely missing your comment in #1685 - by scafall we mean https://github.com/buildpacks-community/scafall

[sarthaksarthak9]

@natalieparellano is this issue still up?