Custom Command chapter updates

[NotTheDr01ds]

• Implements Add def --env to Custom Commands chapter #1552 by adding information on def --env to the Custom Commands chapter. Moves some of it from the Environment chapter, but links between the two.
• The "Command Signature" section was misplaced below quite a few other topics. Moved it into the Custom Commands chapter.
• Extensive rework/rewording of much of the Custom Commands chapter.
  • Starts out with an explanation of why we call them "custom commands"
  • (Nit-pick) "Hello, " examples have proper capitalization and punctuation.
  • Adds information on implicit return. Before someone asks (because I did) - No, Rust does not have implicit return. It simply enforces expressions, which we don't.
  • Adds additional information and examples on return values - match, early return, ignore, and returning null.
  • Adds information on invalid command names
  • Groups all parameter information into an H2 for ToC purposes.
  • Oh yeah, adds a ToC since this is a long chapter
  • Adds information on multi-parameter commands
  • Adds information on non-type shapes that can be used for annotations (e.g., directory)
  • Uses multiple names from many countries as greet <name> examples
  • Adds a few "Cool!" callouts for features that should "Wow" the user (e.g., automatic help for custom commands).
  • Updates table examples to current default style.

[132ikl]

Thank you for this, this section has definitely needed some work 😄

[NotTheDr01ds]

@132ikl Thanks! Converting to a draft while I revise based on your suggestions.

[fdncred]

lgtm!