Add linear easing (aka no easing)

lib/utils/easing.lua

@@ -4,6 +4,11 @@
 
 local easing = {}
 
+--- @type ease
+function easing.linear(x)
+    return x
+end
+
 --- @type ease
 function easing.easeInSine(x)
     return 1 - math.cos((x * math.pi) / 2);