Allow dashes in experimental option identifiers (#16093)

# Description

In #16028 I also added a test to check that identifiers are valid to
ensure that we have consistency there. But I only checked for
alphanumeric strings as identifiers. It doesn't allow underscores or
dashes. @Bahex used in his PR about #15682 a dash to separate words. So
expanded the test to allow that.

# User-Facing Changes

None.

# Tests + Formatting

The `assert_identifiers_are_valid` now allows dashes.

# After Submitting

The tests in #15682 should work then.

crates/nu-experimental/src/options/mod.rs

@@ -75,7 +75,7 @@ mod tests {
             assert!(first.is_lowercase());
 
             for char in chars {
-                assert!(char.is_alphanumeric());
+                assert!(char.is_alphanumeric() || char == '-');
                 if char.is_alphabetic() {
                     assert!(char.is_lowercase());
                 }