Support tag text objects in xml / htmlmixed mode.

[howardjing]

User can use t to operate on tag text objects. For example, given the
following html:

<div>
  <span>hello world!</span>
</div>

If the user's cursor (denoted by █) is inside "hello world!":

<div>
  <span>hello█world!</span>
</div>

And they enter dit (delete inner tag), then the text inside the
enclosing tag is deleted -- the following is the expected result:

<div>
  <span></span>
</div>

If they enter dat (delete around tag), then the surrounding tags are
deleted as well:

<div>
</div>

This logic depends on the following:

• mode/xml/xml.js
• addon/fold/xml-fold.js
• editor is in htmlmixedmode / xml mode

Caveats

This is NOT a 100% accurate implementation of vim tag text objects.
For example, the following cases noop / are inconsistent with vim
behavior:

• Does not work inside comments:

  <!-- <div>broken</div> -->
  

• Does not work when tags have different cases:

  <div>broken</DIV>
  

• Does not work when inside a broken tag:

  <div><brok><en></div>
  

This addresses #3828.