Update "when" commands are available in certain contexts

[emackey]

This refines when import (GLB-to-gltf) and export (gltf-to-GLB) commands appear in certain menus, and generally tries to make a cleaner distinction between registerCommand vs. registerTextEditorCommand.

• Import (convert .glb to .gltf) and export (convert .gltf to .glb) are now available from VSCode's file explorer window context menu, and all operate on the chosen file (not the active editor, as the "export" command previously did).
• glTF-related commands shouldn't appear in context menus of files without .gltf/.glb extensions.
• Preview/Inspect/DataURI commands are no longer available in the title-bar context menu, because they only operate on the active editor. They are available from the editor text context menu, the command pallette, and still assigned the same keyboard shortcuts as before.

Superscedes #225.