Auto merge of #13241 - epage:syntax, r=Muscraft

feat(embedded): Add multiple experimental manifest syntaxes

### What does this PR try to resolve?

As syntax discussions for "cargo script" are on-going, this allows us to experiment with a couple of them so we can see how they work in practice.

This is missing the line-prefix syntax as we decide how we want to separate blocks for it.

While doing this, I removed the previous doc-comment syntax.  This was left in for transition purposes.  With where discussions are going, its unlikely we'll go back to that syntax.

### How should we test and review this PR?

### Additional information

rust-lang/rfcs#3503

https://rust-lang.zulipchat.com/#narrow/stream/213817-t-lang/topic/Syntax.20for.20embedded.20tooling.20metadata

Author: bors

Cargo.lock

@@ -91,7 +91,6 @@ sha2 = "0.10.8"
 shell-escape = "0.1.5"
 supports-hyperlinks = "2.1.0"
 snapbox = { version = "0.4.15", features = ["diff", "path"] }
-syn = { version = "2.0.46", features = ["extra-traits", "full"] }
 tar = { version = "0.4.40", default-features = false }
 tempfile = "3.9.0"
 thiserror = "1.0.56"
@@ -179,7 +178,6 @@ opener.workspace = true
 os_info.workspace = true
 pasetors.workspace = true
 pathdiff.workspace = true
-pulldown-cmark.workspace = true
 rand.workspace = true
 regex.workspace = true
 rusqlite.workspace = true
@@ -192,7 +190,6 @@ serde_json = { workspace = true, features = ["raw_value"] }
 sha1.workspace = true
 shell-escape.workspace = true
 supports-hyperlinks.workspace = true
-syn.workspace = true
 tar.workspace = true
 tempfile.workspace = true
 time.workspace = true