[proposed API][builtins] Theia misses proposed API: ExternalUriOpener

[alvsan09]

Bug Description:

The following exception was encountered while activating built-in extensions in theia coming from vscode 1.77.2

2023-05-11T19:40:54.698Z root ERROR [hosted-plugin: 9798] Activating extension 'Simple Browser' failed: TypeError: vscode.window.registerExternalUriOpener is not a function
The extension uses the proposed API: ExternalUriOpener

Steps to Reproduce:

simple-browser-1.77.2-next.e344f1f539a.zip

1. Download and change the extension of the file above to vsix
2. Install it in 'theia', either via the Extensions view or by placing it inside the plugins folder
3. Attempt to activate the extension by running the command Simple Browser: Show
4. You will see a toast message stating that the activation of the Simple Browser (built-in) extension failed.

Additional Information

2023-05-11T19:40:54.698Z root ERROR [hosted-plugin: 9798] Activating extension 'Simple Browser' failed: TypeError: vscode.window.registerExternalUriOpener is not a function
    at activate (/home/lmcalvs/git/vscode4/extensions/simple-browser/out/extension.js:49:46)
    at PluginManagerExtImpl.startPlugin (/home/lmcalvs/git/theia2/packages/plugin-ext/lib/plugin/plugin-manager.js:310:81)
    at processTicksAndRejections (node:internal/process/task_queues:96:5)
    at async /home/lmcalvs/git/theia2/packages/plugin-ext/lib/plugin/plugin-manager.js:219:21
    at async PluginManagerExtImpl.$activatePlugin (/home/lmcalvs/git/theia2/packages/plugin-ext/lib/plugin/plugin-manager.js:271:13)
    at async Promise.all (index 14)
    at async PluginManagerExtImpl.activateBySingleEvent (/home/lmcalvs/git/theia2/packages/plugin-ext/lib/plugin/plugin-manager.js:266:9)
    at async PluginManagerExtImpl.$activateByEvent (/home/lmcalvs/git/theia2/packages/plugin-ext/lib/plugin/plugin-manager.js:250:13)
    at async PluginManagerExtImpl.$start (/home/lmcalvs/git/theia2/packages/plugin-ext/lib/plugin/plugin-manager.js:148:9)
    at async RpcProtocol.handleRequest (/home/lmcalvs/git/theia2/packages/core/lib/common/message-rpc/rpc-protocol.js:167:28)

• Operating System:
• Theia Version:

[alvsan09]

Fixed by: #12539