Upgrade to vue2.7 and remove @vue/composition-api.

[rtibbles]

Summary

• Upgrades KDS to the latest rc that uses Vue 2.7
• Upgrades Vue to latest 2.7 and the associated template compiler
• Removes @vue/composition-api plugin completely
• Upgrades @vueuse/core to the latest version compatible with Vue 2
• Updates all named imports from @vue/composition-api to vue
• Removes registration of the @vue/composition-api plugin
• Updates our webpack configuration as apparently the janky dev/prod module exposure that was used in @vue/composition-api is now being used in vue2.7
• Regenerates the core API to account for the removal of the @vue/composition-api plugin
• Updates the moduleMapping for core API migration to convert all references to vue instead

References

Fixes #9538

Reviewer guidance

Run the devserver, make sure to interact with both composable and options API based code.
I am fairly sure the last time we upgraded @vueuse/core we reverted it because it wasn't compatible with Vue 2.6, but it is compatible with Vue 2.7 - if you can find any evidence or discussion about this to confirm, that would be great.
Make sure that all globally registered things (like vue-intl methods, core theme etc) are working fine.

Question - why do we have two nearly (but not quite) identical copies of the search composable tests, one in learn, one in kolibri-common?

[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-0.18.0a0.dev0_git.121.g9ac1c5ad.pex
Windows Installer (EXE)	kolibri-0.18.0a0.dev0+git.121.g9ac1c5ad-windows-setup-unsigned.exe
Debian Package	kolibri_0.18.0a0.dev0+git.121.g9ac1c5ad-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.18.0a0.dev0+git.121.g9ac1c5ad.dmg
Android Package (APK)	kolibri-0.18.0a0.dev0+git.121.g9ac1c5ad-0.1.4-debug.apk
TAR file	kolibri-0.18.0a0.dev0+git.121.g9ac1c5ad.tar.gz
WHL file	kolibri-0.18.0a0.dev0+git.121.g9ac1c5ad-py2.py3-none-any.whl

[bjester]

LGTM -- changes make sense

[AlexVelezLl]

Thank you @rtibbles!