Add an error for a slot name starts with a digit

src/compile/nodes/Slot.ts

@@ -34,6 +34,13 @@ export default class Slot extends Element {
 						message: `default is a reserved word — it cannot be used as a slot name`
 					});
 				}
+
+				if (/^[0-9]/.test(slot_name)) {
+					component.error(attr, {
+						code: `invalid-slot-name`,
+						message: `<slot> name cannot start with a digit`
+					});
+				}
 			}
 
 			// TODO should duplicate slots be disallowed? Feels like it's more likely to be a
@@ -70,4 +77,4 @@ export default class Slot extends Element {
 
 		return null;
 	}
-}
+}

test/validator/samples/slot-name-starts-with-a-digit/errors.json

@@ -0,0 +1,15 @@
+[{
+	"code": "invalid-slot-name",
+	"message": "<slot> name cannot start with a digit",
+	"start": {
+		"line": 2,
+		"column": 7,
+		"character": 13
+	},
+	"end": {
+		"line": 2,
+		"column": 21,
+		"character": 27
+	},
+	"pos": 13
+}]

test/validator/samples/slot-name-starts-with-a-digit/input.svelte

@@ -0,0 +1,3 @@
+<div>
+	<slot name="1header" />
+</div>