changedProperties check if name is actually a @property or @state

[Christian24]

Hello,

In Lit a lot of lifecycle callbacks use a Map called changedProperties which holds all properties that changed during the update. For example changedProperties.has('foo') says if property foo changed. If you rename the property foo now, you also need to rename it where ever you use it in changedProperties. This is error prone and a rule that would detect property names that are not actual properties would help ease the pain a little bit.