custom-editor: changeset unnecessarily pulls '@theia/editor-preview'

[vince-fugnitto]

Bug Description:

The change #8910 adds an unnecessary coupling to @theia/editor-preview, and means that downstream applications cannot easily opt-out of using @theia/editor-preview as part of their application. This is considered a breaking change.

I think in this particular instance the coupling can be avoided:

theia/packages/plugin-ext/src/main/browser/custom-editors/custom-editor-opener.tsx

Line 18 in fd91f21

import { PreviewEditorOpenerOptions } from '@theia/editor-preview/lib/browser';

theia/packages/plugin-ext/src/main/browser/custom-editors/custom-editor-opener.tsx

Line 45 in fd91f21

canHandle(uri: URI, options?: PreviewEditorOpenerOptions): number {