Fix template decorators to work without parentheses

This fixes an issue where template decorators like @app.template_filter
don't work when used without parentheses. The fix allows the decorators
to be used in both ways:

- @app.template_filter
- @app.template_filter()

The following decorators have been fixed:
- template_filter, template_test, template_global in Flask
- app_template_filter, app_template_test, app_template_global in Blueprint

Fixes #5729

tests/test_decorator_no_parens.py

@@ -0,0 +1,77 @@
+import pytest
+
+import flask
+
+
+def test_template_filter_no_parens(app):
+    """Test that @app.template_filter works without parentheses."""
+    @app.template_filter
+    def double(x):
+        return x * 2
+
+    assert "double" in app.jinja_env.filters
+    assert app.jinja_env.filters["double"] == double
+    assert app.jinja_env.filters["double"](2) == 4
+
+
+def test_template_test_no_parens(app):
+    """Test that @app.template_test works without parentheses."""
+    @app.template_test
+    def is_even(x):
+        return x % 2 == 0
+
+    assert "is_even" in app.jinja_env.tests
+    assert app.jinja_env.tests["is_even"] == is_even
+    assert app.jinja_env.tests["is_even"](2) is True
+    assert app.jinja_env.tests["is_even"](3) is False
+
+
+def test_template_global_no_parens(app):
+    """Test that @app.template_global works without parentheses."""
+    @app.template_global
+    def get_answer():
+        return 42
+
+    assert "get_answer" in app.jinja_env.globals
+    assert app.jinja_env.globals["get_answer"] == get_answer
+    assert app.jinja_env.globals["get_answer"]() == 42
+
+
+def test_blueprint_app_template_filter_no_parens(app):
+    """Test that @blueprint.app_template_filter works without parentheses."""
+    bp = flask.Blueprint("test_bp", __name__)
+
+    @bp.app_template_filter
+    def triple(x):
+        return x * 3
+
+    app.register_blueprint(bp)
+    assert "triple" in app.jinja_env.filters
+    assert app.jinja_env.filters["triple"](3) == 9
+
+
+def test_blueprint_app_template_test_no_parens(app):
+    """Test that @blueprint.app_template_test works without parentheses."""
+    bp = flask.Blueprint("test_bp", __name__)
+
+    @bp.app_template_test
+    def is_odd(x):
+        return x % 2 == 1
+
+    app.register_blueprint(bp)
+    assert "is_odd" in app.jinja_env.tests
+    assert app.jinja_env.tests["is_odd"](3) is True
+    assert app.jinja_env.tests["is_odd"](2) is False
+
+
+def test_blueprint_app_template_global_no_parens(app):
+    """Test that @blueprint.app_template_global works without parentheses."""
+    bp = flask.Blueprint("test_bp", __name__)
+
+    @bp.app_template_global
+    def get_pi():
+        return 3.14
+
+    app.register_blueprint(bp)
+    assert "get_pi" in app.jinja_env.globals
+    assert app.jinja_env.globals["get_pi"]() == 3.14