feat(i18n): try to resolve $t from vue global properties

[m0ksem]

Beside our custom config, let's try to resolve $t from getCurrentInstance() global properties.
If $t is preset, try to resolve key like vuestic:${key}.

Add this feature to docs. This is much easier way to work with i18n.

In case if $t is not found, use the same approach we use rn.