Move $.byteLength and $.trimByteLength to new module 'mediawiki.String'

These methods do not belong on the jQuery object. And to resolve
T185948, we need to also add codePointLength and trimCodePointLength,
and this new module seems like a good place to put them.

There is no `mw.String` global, this module has to be used via `require()`.

Deprecations:
* Function `$.byteLength` (from module 'jquery.byteLength') is
  deprecated, use `require( 'mediawiki.String' ).byteLength` instead.
* Function `$.trimByteLength` (from module 'jquery.byteLimit') is
  deprecated, use `require( 'mediawiki.String' ).trimByteLength` instead.
* Module 'jquery.byteLength' is deprecated, use 'mediawiki.String' instead.

Note that `$.fn.byteLimit` and the 'jquery.byteLimit' module are not
deprecated.

Change-Id: I2501a79efee644e5f4a9f5c977fe49c8c05c6eb3