Fix bug introduced by nushell#13595 (nushell#13658)

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# Description
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@devyn found that nushell#13595, which
made ranges be type-checked at parse time, introduced a bug that caused
`../foo` to be parsed as a string rather than a command call. This was
caused by `parse_range` returning a `Some` despite there being parse
errors (`/foo` doesn't match `SyntaxShape::Number`). To go back to the
old behavior, `parse_range` now returns `None` anytime there's any parse
errors met while parsing the range.

Unfortunately, this means that something like `..$foo` will be parsed as
a string if `$foo` isn't defined and as a range if it is defined. That
was the behavior before nushell#13595, and it should probably be fixed at some
point, but I'm just trying to quickly fix the bug.

# User-Facing Changes
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Things should go back to the way they were before nushell#13595, except the
type-checking stuff from that PR is still here.

# Tests + Formatting
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Added a test. Reverted another test that tests that `0..<$day` is parsed
successfully as a string if the variable isn't defined.

# After Submitting
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crates/nu-parser/src/parser.rs

@@ -1594,6 +1594,7 @@ pub fn parse_number(working_set: &mut StateWorkingSet, span: Span) -> Expression
 
 pub fn parse_range(working_set: &mut StateWorkingSet, span: Span) -> Option<Expression> {
     trace!("parsing: range");
+    let starting_error_count = working_set.parse_errors.len();
 
     // Range follows the following syntax: [<from>][<next_operator><next>]<range_operator>[<to>]
     //   where <next_operator> is ".."
@@ -1715,6 +1716,10 @@ pub fn parse_range(working_set: &mut StateWorkingSet, span: Span) -> Option<Expr
         (None, span)
     };
 
+    if working_set.parse_errors.len() != starting_error_count {
+        return None;
+    }
+
     let operator = RangeOperator {
         inclusion,
         span: range_op_span,

crates/nu-parser/tests/test_parser.rs

@@ -1828,23 +1828,13 @@ mod range {
     }
 
     #[test]
-    fn vars_read_as_units() {
+    fn vars_not_read_as_units() {
         let engine_state = EngineState::new();
         let mut working_set = StateWorkingSet::new(&engine_state);
 
         let _ = parse(&mut working_set, None, b"0..<$day", true);
 
-        assert!(
-            working_set.parse_errors.len() == 1,
-            "Errors: {:?}",
-            working_set.parse_errors
-        );
-        let err = &working_set.parse_errors[0].to_string();
-        assert!(
-            err.contains("Variable not found"),
-            "Expected variable not found error, got {}",
-            err
-        );
+        assert!(working_set.parse_errors.is_empty());
     }
 
     #[rstest]
@@ -1878,6 +1868,26 @@ mod range {
             err
         );
     }
+
+    #[test]
+    fn dont_mess_with_external_calls() {
+        let engine_state = EngineState::new();
+        let mut working_set = StateWorkingSet::new(&engine_state);
+        working_set.add_decl(Box::new(ToCustom));
+
+        let result = parse(&mut working_set, None, b"../foo", true);
+
+        assert!(
+            working_set.parse_errors.is_empty(),
+            "Errors: {:?}",
+            working_set.parse_errors
+        );
+        let expr = &result.pipelines[0].elements[0].expr.expr;
+        assert!(
+            matches!(expr, Expr::ExternalCall(..)),
+            "Should've been parsed as a call"
+        );
+    }
 }
 
 #[cfg(test)]