Make get_full_help take &dyn Command (#12903)

# Description
Changes `get_full_help` to take a `&dyn Command` instead of multiple
arguments (`&Signature`, `&Examples` `is_parser_keyword`). All of these
arguments can be gathered from a `Command`, so there is no need to pass
the pieces to `get_full_help`.

This PR also fixes an issue where the search terms are not shown if
`--help` is used on a command.

tests/repl/test_engine.rs

@@ -54,8 +54,7 @@ fn in_and_if_else() -> TestResult {
 
 #[test]
 fn help_works_with_missing_requirements() -> TestResult {
-    let expected_length = "70";
-    run_test(r#"each --help | lines | length"#, expected_length)
+    run_test(r#"each --help | lines | length"#, "72")
 }
 
 #[test]
@@ -65,12 +64,12 @@ fn scope_variable() -> TestResult {
         "int",
     )
 }
+
 #[rstest]
 #[case("a", "<> nothing")]
 #[case("b", "<1.23> float")]
 #[case("flag1", "<> nothing")]
 #[case("flag2", "<4.56> float")]
-
 fn scope_command_defaults(#[case] var: &str, #[case] exp_result: &str) -> TestResult {
     run_test(
         &format!(