Upgrade/remove dependency `core-js`

[FeBe95]

When installing vue-functional-calendar a warning is shown which states that core-js@2.6.12 is no longer maintained:

npm WARN deprecated core-js@2.6.12: core-js@<3.23.3 is no longer maintained and not recommended for usage due
to the number of issues. Because of the V8 engine whims, feature detection in old core-js versions could cause
a slowdown up to 100x even if nothing is polyfilled. Some versions have web compatibility issues. Please, upgrade
your dependencies to the actual version of core-js.

I looked through the code of vue-functional-calendar, but I couldn't find any usages of core-js.

• If core-js isn't used at all, it should be removed from the dependency list.
• If core-js is used somewhere, it should be upgraded to the newest core-js version.
• If core-js is only used during development, it should be moved to devDependencies.

FYI: The package was introduced in this commit: 8a2f06b.