Assorted README.md updates

[YellowApple]

This PR updates the various examples in README.md to reflect the need for a dependencies key under pkgx (per the test cases under fixtures/). I was banging my head on my keyboard for a second wondering why a pkgx.yaml with

pkgx:
  typst.app: 0.13.0

wasn't actually adding typst to my $PATH until I checked the relevant test case and noticed that the syntax changed to

dependencies:
  typst.app: 0.13.0

Hopefully this'll spare folks some of that frustration :)

While I was at it, I noticed that the TOML test cases (Cargo.toml, pixi.toml, the various variations on pyproject.toml) specify # --- fences around the frontmatter, so that's now reflected in the README.md examples as well.

I also removed the warning about command substitutions in environment variables, since those are no longer possible as of e73e559 (#841, #842). Tested both BAD_ENV: $(pwd) and BAD_ENV: \pwd`to confirm that both still throw errors (though it'd probably be a good idea to havedev` throw a specific "don't try running commands" error in both cases).

EDIT: also updated the "one-liner" example, addressing #22 as far as README.md goes.

[mxcl]

Thanks. Sorry for the delay in merging.