Make miradorImageToolsPlugin the default export for src/index.

[cbeer]

No description provided.

[marlo-longley]

@cbeer I just tried to rebase this and ran into the eslint rule violation import/no-named-as-default which prevents a name conflict between a named export and a default export with the same identifier. Apparently the best practice is to used name exports. I realize our other plugins use default exports. If consistency was the goal with this PR, we could disable the eslint rule, or bring the other plugins (share, download), over to named exports.