Add an add template filter (#109884)

* Addition of add filter

This change adds an `add` filter, the addition equivalent of the existing `multiply` filter.

* Test for add filter

* Update test_template.py

* Update tests/helpers/test_template.py

---------

Co-authored-by: Erik Montnemery <erik@montnemery.com>

homeassistant/helpers/template.py

@@ -1888,6 +1888,17 @@ def multiply(value, amount, default=_SENTINEL):
         return default
 
 
+def add(value, amount, default=_SENTINEL):
+    """Filter to convert value to float and add it."""
+    try:
+        return float(value) + amount
+    except (ValueError, TypeError):
+        # If value can't be converted to float
+        if default is _SENTINEL:
+            raise_no_default("add", value)
+        return default
+
+
 def logarithm(value, base=math.e, default=_SENTINEL):
     """Filter and function to get logarithm of the value with a specific base."""
     try:
@@ -2728,6 +2739,7 @@ class TemplateEnvironment(ImmutableSandboxedEnvironment):
         self.add_extension("jinja2.ext.loopcontrols")
         self.filters["round"] = forgiving_round
         self.filters["multiply"] = multiply
+        self.filters["add"] = add
         self.filters["log"] = logarithm
         self.filters["sin"] = sine
         self.filters["cos"] = cosine