Merge vscode-workspace-resource and vscode-extension-resource

[mjbvz]

Problem
Webview API proposal (#43713) introduces two resource schemes for loading local files:

• vscode-workspace-resource: Resources from the user's workspace. This can also be overridden by localResourceRoots
• vscode-extension-resource: This allows reading from extension's install directory

However this scheme prevents cross origin requests between extension resources and workspace resources. vscode-extension-resource also currently allows reading any extension's resources, which may not be safe (if, for example, some crazy extension stores user creds in the extension's install directory)

Proposal
Replace these two with a new vscode-resource scheme. This scheme would allow access to files within localResourceRoots. If not specified, localResourceRoots would include:

• Currently opened workspaces folders
• The extension's install directory

Extensions could override this with localResourceRoots

[ramya-rao-a]

Moving to April as this is essentially part of #43713 which is for April