Improve keymap json schema (#23044)

Also:

* Adds `impl_internal_actions!` for deriving the `Action` trait without
registering.

* Removes some deserializers that immediately fail in favor of
`#[serde(skip)]` on fields where they were used. This also omits them
from the schema.

Release Notes:

- Keymap settings file now has more JSON schema information to inform
`json-language-server` completions and info, particularly for actions
that take input.

crates/languages/Cargo.toml

@@ -56,6 +56,7 @@ project.workspace = true
 regex.workspace = true
 rope.workspace = true
 rust-embed.workspace = true
+schemars.workspace = true
 serde.workspace = true
 serde_json.workspace = true
 settings.workspace = true