fix: viewer is not using exts prop (ref #16)

[rodumani]

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Description

Although PR #17 has added exts property in Viewer, it is not used anywhere.

Despite of it, examples of viewer storybook are working because its Editor has initialized extensions at editor storybook. Extensions of viewer is not configured by viewer storybook itself. If you try viewer storybook without editor storybook, extensions are not working.

So this PR fix Viewer to use its exts property.

Also, scrollSync extension in viewer storybook is causing error.

Cannot read property 'get' of undefined
TypeError: Cannot read property 'get' of undefined
    at scrollSyncExtension (http://localhost:6006/vendors~main.566b849c84711d1f2149.bundle.js:188532:18)
    at http://localhost:6006/vendors~main.566b849c84711d1f2149.bundle.js:151695:15
    at Array.forEach (<anonymous>)
    at ExtManager.applyExtension (http://localhost:6006/vendors~main.566b849c84711d1f2149.bundle.js:151686:17)
    at new ToastUIEditorViewer (http://localhost:6006/vendors~main.566b849c84711d1f2149.bundle.js:154061:26)
    at Function.factory (http://localhost:6006/vendors~main.566b849c84711d1f2149.bundle.js:159353:21)
    at VueComponent.mounted (http://localhost:6006/main.1574541a3aa60a44d3a5.hot-update.js:64:69)
    at invokeWithErrorHandling (http://localhost:6006/vendors~main.566b849c84711d1f2149.bundle.js:196508:57)
    at callHook (http://localhost:6006/vendors~main.566b849c84711d1f2149.bundle.js:198825:7)
    at Object.insert (http://localhost:6006/vendors~main.566b849c84711d1f2149.bundle.js:197773:7)

Because viewer does not need scrollSync extension, this PR removes it from viewer storybook.

In addition, as minor change, this PR removes unused import from storybooks.

[sohee-lee7]

Thank you for contribution. 👍 👍 👍