`tea --help` lists available targets

[miles170]

Closes #145.

Following is the help generated from teaxyz/cli/README.md

Since the target description is muliptline, I haven't find a good way to display the description.

$ tea run --help
usage:
  tea [-xdX] [flags] [+package~x.y] [file|URL|target|cmd|interpreter] -- [arg…]

modes:                                            magical?
  --exec,-x         execute
  --dump,-d         dump
  -X                magic execute
  𝑜𝑚𝑖𝑡𝑡𝑒𝑑           infer operation                  ✨

flags:
  --env,-E          inject virtual environment       ✨
  --sync,-S         sync pantries, etc. first        ✨
  --magic=no,-m     disable magic
  --verbose,-v      eg. tea -vv
  --silent,-s       no chat, no errors
  --cd,-C           change directory first

more:
  tea -vh
  open github.com/teaxyz/cli

targets:
  getting-started
    tea is a standalone binary so, if you like, you can just download it
    yourself. For a little more magic†, however, we recommend our installer:

[what-the-diff[bot]]

• Added a new target to the executable markdown file
• Updated import-map.json with an additional dependency
• Refactored app code for better readability and maintainability (no functional changes)

[mxcl]

Sorry this has remained unmerged. I thought we were still debating how to identify if a block should be treated as a target or not, but I see this is ready and we can figure that out later.

[mxcl]

We have removed executable markdown from tea deeming it out of scope. Sorry for your wasted work 😔