Add return types to all interface methods in Zowe Explorer API

[t1m0thyj]

Zowe Explorer API defines many interfaces containing methods, some of which do not have return types defined.

For example, many of the methods on the IZoweTree interface are lacking return types:
https://github.com/zowe/zowe-explorer-vscode/blob/next/packages/zowe-explorer-api/src/tree/IZoweTree.ts

Adding return types could be considered a breaking change but should improve the experience for developers/extenders. For example, when calling ZoweTreeProvider.addFavorite it'd be nice to have a return type Promise<void> displayed by IntelliSense instead of any 😋

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[traeok]

Something to consider as part of adding return types: do we intend on supporting asynchronous types for some functions?

For example, the rename function could be typed as follows to allow both asynchronous and synchronous operations:

/**
 * Rename the node. Begins a dialog.
 * @param the node to be renamed
 */
rename(node: IZoweTreeNode): void | PromiseLike<void>;