Add readonly flag to properties

[bennypowers]

It would be helpful to mark a class field or property as readonly (i.e. getter only or defined as writable: false)

{
  "name": "elements",
  "type": "readonly ApolloElementElement[]",
  "readonly": true,
  "attribute": false,
  "summary": "List of all ApolloElements registered to this client."
},

[aFarkas]

This could be even useful with CSS props.

[hsablonniere]

I have a map component and 2 marker components that expose 3 readonly properties, I used to annotate them like this:

/** @readonly */
this.size = [16, 16];
/** @readonly */
this.anchor = [8, 8];
/** @readonly */
this.tooltip = [0, 0];

[hsablonniere]

I decided to move those to getters (without setters) and I'll create a CEM analyzer plugin that used this "getter but no setter" to identify fields as readonly.