vscode: add full support for Diagnostic.code

[vince-fugnitto]

Feature Description:

The framework currently only has partial support for Diagnostic.code:

theia/packages/plugin/src/theia.d.ts

Lines 8070 to 8075 in 62ce786

	/**
	* A code or identifier for this diagnostics. Will not be surfaced
	* to the user, but should be used for later processing, e.g. when
	* providing {@link CodeActionContext code actions}.
	*/
	code?: string | number;

We should update to have full support according to the VS Code API:

/**
 * A code or identifier for this diagnostic.
 * Should be used for later processing, e.g. when providing {@link CodeActionContext code actions}.
 */
code?: string | number | {
    /**
     * A code or identifier for this diagnostic.
     * Should be used for later processing, e.g. when providing {@link CodeActionContext code actions}.
     */
    value: string | number;

    /**
     * A target URI to open with more information about the diagnostic error.
     */
    target: Uri;
};