make command signature parsing more strict (#14309)

# User-Facing Changes

The parser now errors on more invalid command signatures:

```nushell
# expected parameter or flag
def foo [ bar: int: ] {}

# expected type
def foo [ bar: =  ] {}
def foo [ bar: ] {}

# expected default value
def foo [ bar = ] {}
```

crates/nu-command/tests/commands/def.rs

@@ -2,6 +2,13 @@ use nu_test_support::nu;
 use nu_test_support::playground::Playground;
 use std::fs;
 
+#[test]
+fn def_with_trailing_comma() {
+    let actual = nu!("def test-command [ foo: int, ] { $foo }; test-command 1");
+
+    assert!(actual.out == "1");
+}
+
 #[test]
 fn def_with_comment() {
     Playground::setup("def_with_comment", |dirs, _| {
@@ -72,6 +79,13 @@ fn def_errors_with_comma_before_equals() {
     assert!(actual.err.contains("expected parameter"));
 }
 
+#[test]
+fn def_errors_with_colon_before_equals() {
+    let actual = nu!("def test-command [ foo: = 1 ] {}");
+
+    assert!(actual.err.contains("expected type"));
+}
+
 #[test]
 fn def_errors_with_comma_before_colon() {
     let actual = nu!("def test-command [ foo, : int ] {}");
@@ -85,7 +99,6 @@ fn def_errors_with_multiple_colons() {
     assert!(actual.err.contains("expected type"));
 }
 
-#[ignore = "This error condition is not implemented yet"]
 #[test]
 fn def_errors_with_multiple_types() {
     let actual = nu!("def test-command [ foo:int:string ] {}");
@@ -93,6 +106,20 @@ fn def_errors_with_multiple_types() {
     assert!(actual.err.contains("expected parameter"));
 }
 
+#[test]
+fn def_errors_with_trailing_colon() {
+    let actual = nu!("def test-command [ foo: int: ] {}");
+
+    assert!(actual.err.contains("expected parameter"));
+}
+
+#[test]
+fn def_errors_with_trailing_default_value() {
+    let actual = nu!("def test-command [ foo: int = ] {}");
+
+    assert!(actual.err.contains("expected default value"));
+}
+
 #[test]
 fn def_errors_with_multiple_commas() {
     let actual = nu!("def test-command [ foo,,bar ] {}");