Allow component interpolation without root element

[danimoh]

Component interpolation without a root element can now be enabled
by setting the tag prop to false.
Let me know if you prefer something else then value false, for example
a keyword like "none".
Value false would need to be specified as :tag="false", a keyword as
tag="none" which might read a little nicer.

Fixes #876

[kazupon]

Thank you for your PR!
This fix is ​​an enhancement, but we need to keep backward compatible.

<i18n> component tag prop is currently string type only.
https://kazupon.github.io/vue-i18n/api/#i18n-functional-component

My recommendation is that backwards compatibility can be maintained by also supporting boolean types. when passed boolean value (false or true), returning children.
If tag prop is nothing, it should be span.

The same goes for <i18n-n> component.

[danimoh]

That makes sense.
What would a value of true stand for though?
Should the default be used for true which is a span?

Would you want to work on the additional changes to the documentation, i18n-n and support for true yourself?
That would likely be faster then me trying to figure it out.
But if you want, I can help of course.

[kazupon]

What would a value of true stand for though?

if tag prop value is true, it should be span, else false value, it should be returned children.

Should the default be used for true which is a span?

if tag prop is nothing, it should be true same.

Would you want to work on the additional changes to the documentation, i18n-n and support for true yourself?

I'm busy developing for vue-i18n-next, so I'm glad if you could help. 🙏

[danimoh]

I changed the behavior for true as discussed and added support for i18n-n components without root element.
I also added tests and documentation for this feature.
I hope I built the docs correctly, please have a look.

[codecov-io]

Codecov Report

Merging #878 into v8.x will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             v8.x     #878   +/-   ##
=======================================
  Coverage   96.38%   96.38%           
=======================================
  Files          10       10           
  Lines         885      886    +1     
=======================================
+ Hits          853      854    +1     
  Misses         32       32           

Impacted Files	Coverage Δ
src/components/interpolation.js	100.00% <ø> (ø)
src/components/number.js	100.00% <ø> (ø)

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[kazupon]

Thanks!