replace rustlings-macros with build script

This turns rustlings into a single crate, which is less complicated to
publish releases for. A follow-up step might be to remove the workspace
configuration.

The motivation was this discussion on Zulip:
https://rust-lang.zulipchat.com/#narrow/channel/334454-t-lang-docs.2Frustlings/topic/Trusted.20publishing/near/638396712

Cargo.lock

@@ -434,8 +434,8 @@ dependencies = [
  "clap",
  "crossterm",
  "notify",
+ "quote",
  "rustix",
- "rustlings-macros",
  "serde",
  "serde_json",
  "shlex",
@@ -444,15 +444,6 @@ dependencies = [
  "wait-timeout",
 ]
 
-[[package]]
-name = "rustlings-macros"
-version = "6.5.0"
-dependencies = [
- "quote",
- "serde",
- "toml",
-]
-
 [[package]]
 name = "same-file"
 version = "1.0.6"