Fixes #7604: Add filter modifier dropdowns for advanced lookup operators

Implements dynamic filter modifier UI that allows users to select lookup operators
(exact, contains, starts with, regex, negation, empty/not empty) directly in filter
forms without manual URL parameter editing.

Supports filters for all scalar types and strings, as well as some
related object filters. Explicitly does not support filters on fields
that use APIWidget. That has been broken out in to follow up work.

**Backend:**
- FilterModifierWidget: Wraps form widgets with lookup modifier dropdown
- FilterModifierMixin: Auto-enhances filterset fields with appropriate lookups
- Extended lookup support: Adds negation (n), regex, iregex, empty_true/false lookups
- Field-type-aware: CharField gets text lookups, IntegerField gets comparison operators, etc.

**Frontend:**
- TypeScript handler syncs modifier dropdown with URL parameters
- Dynamically updates form field names (serial → serial__ic) on modifier change
- Flexible-width modifier dropdowns with semantic CSS classes

netbox/project-static/styles/transitional/_forms.scss

@@ -32,3 +32,11 @@ form.object-edit {
     border: 1px solid $red;
   }
 }
+
+// Filter modifier dropdown sizing
+.modifier-select {
+  min-width: 10rem;
+  max-width: 15rem;
+  width: auto;
+  white-space: nowrap;
+}