Allow duration to be added to date (#14295)

# Description

Fixes #14294 - Turned out to be a whole lot easier than I expected, but
please double-check me on this, since it's an area I haven't been in
before.

# User-Facing Changes

Allow date to be added to a duration type.

# Tests + Formatting

Tests added:

* Duration + Date is allowed
* Duration - Date is not allowed

crates/nu-parser/src/type_check.rs

@@ -88,6 +88,7 @@ pub fn math_result_type(
                 (Type::Float, Type::Number) => (Type::Number, None),
                 (Type::String, Type::String) => (Type::String, None),
                 (Type::Date, Type::Duration) => (Type::Date, None),
+                (Type::Duration, Type::Date) => (Type::Date, None),
                 (Type::Duration, Type::Duration) => (Type::Duration, None),
                 (Type::Filesize, Type::Filesize) => (Type::Filesize, None),