Make LitElement's property accessors creation and metadata easily customizable

[sorvell]

Initiative / goal

LitElement configures reactive properties, but this behavior is currently difficult to extend and customize. Making property creation and metadata easily customizable would unlock many use specific cases that users have asked for but that we do not want to burden everyone with by adding the behavior to the core library.

Scope

• Make property metadata easily customizable.
  • One approach is to create static methods for setting/getting property metadata for a property.
  • This work must also ensure that metadata can continue to be typed correctly in TypeScript.
• Make property accessors easily customizable. While users can currently override static createProperty, the code for making a property reactive is private.
  • One approach is to rename and expose the private _requestUpdate method.
  • This leaves private storage as the only not easily callable work done in createProperty. This could be sugared or left to overriders to deal with.

Acceptance criteria and must have scope

Add a set of tests that prove that the requested use cases can be implemented by adding this features. For example:

• ability to override default property metadata (Add ability to override default converter and default getter/setter #891)
• providing a property validator (Proposal: default/fallback values for properties #905)
• making properties have synchronous effects (may only be part of this) (As a developer I would like to do sync actions on property changes #643)