Support MetadataMap from renderElementJSON

[roblourens]

Notes from Connor

• Maybe there's a subclass like TransferrablePromptMetadata that has a key and value we serialize?

  vscode-prompt-tsx/src/base/results.ts

  Line 12 in bd0623f

  export abstract class PromptMetadata {

• It's serialized here

  vscode-prompt-tsx/src/base/promptRenderer.ts

  Line 948 in bd0623f

  const json: JSONT.PieceJSON = {

  We could look for metadata that's instanceof TransferrablePromptMetadata. Hydrated in fromJSON in that same class.
• Then we could update the MetadataMap type to take a string too and modify its implementation here

  vscode-prompt-tsx/src/base/promptRenderer.ts

  Lines 421 to 424 in bd0623f

  	metadata: {
  	get: ctor => remainingMetadata.find(m => m instanceof ctor) as any,
  	getAll: ctor => remainingMetadata.filter(m => m instanceof ctor) as any,
  	},

Then I can remove the hack I did here https://github.com/microsoft/vscode-copilot/pull/13129/files#diff-5e5185bdca8d65a2172bf8f5e881001e40f9748dff53ade752f6be110073e034R55