vscode: implement support for `webview/context` menu contributions

[vince-fugnitto]

Feature Description:

The goal is to add support for the webview/context menu path which extension authors can reference when declaring their menu contributions.

Additional Information

The vscode.media-preview builtin extension makes use of the menu path:

logger-protocol.ts:113 2023-07-25T18:49:43.379Z root WARN Failed to register a menu item for plugin vscode.media-preview contributed to webview/context {command: 'imagePreview.copyImage', submenu: undefined, alt: undefined, group: undefined, when: "webviewId == 'imagePreview.previewEditor'"} Error: Could not find submenu with id webview/context
    at MenuModelRegistry.getMenuNode (http://localhost:3000/bundle.js:147792:23)
    at http://localhost:3000/packages_plugin-ext_lib_hosted_browser_hosted-plugin_js.js:5826:66
    at Array.forEach (<anonymous>)
    at MenusContributionPointHandler.handle (http://localhost:3000/packages_plugin-ext_lib_hosted_browser_hosted-plugin_js.js:5820:37)
    at http://localhost:3000/packages_plugin-ext_lib_hosted_browser_hosted-plugin_js.js:6563:71
    at pushContribution (http://localhost:3000/packages_plugin-ext_lib_hosted_browser_hosted-plugin_js.js:6458:32)
    at PluginContributionHandler.handleContributions (http://localhost:3000/packages_plugin-ext_lib_hosted_browser_hosted-plugin_js.js:6563:9)
    at HostedPluginSupport.loadContributions (http://localhost:3000/packages_plugin-ext_lib_hosted_browser_hosted-plugin_js.js:577:61)
    at HostedPluginSupport.doLoad (http://localhost:3000/packages_plugin-ext_lib_hosted_browser_hosted-plugin_js.js:488:42)
    at async ProgressService.withProgress (http://localhost:3000/bundle.js:151138:20)