refactor(embedded): Switch to syn for parsing doc comments

The hope is this will result in more resilient comment handling, being
more consistent with rustdoc.

I also hoped for less code but `syn` is doing less than I had expected,
requiring us to copy code over from other parts of rust.  It seems every
proc macro has to do this but there is no guide to it, so they all do it
differently, only covering the cases they thought to test for.

Note that this still won't support `include_str!()`.

Cargo.toml

@@ -67,7 +67,6 @@ pretty_env_logger = "0.4"
 proptest = "1.1.0"
 pulldown-cmark = { version = "0.9.2", default-features = false }
 rand = "0.8.5"
-regex = "1.8.3"
 rustfix = "0.6.0"
 same-file = "1.0.6"
 security-framework = "2.0.0"
@@ -153,7 +152,6 @@ pathdiff.workspace = true
 pretty_env_logger = { workspace = true, optional = true }
 pulldown-cmark.workspace = true
 rand.workspace = true
-regex.workspace = true
 rustfix.workspace = true
 semver.workspace = true
 serde = { workspace = true, features = ["derive"] }