[HTML] Format values of attributes with defined semantics

[YarnSphere]

As briefly discussed in #5479, this issue targets the formatting of HTML attribute values that have defined semantics: for example, in regard to white spaces or commas.

An example of such an attribute is class, where class=" a b c " should be semantically equivalent to class="a b c". However, note that if the user is parsing element.className directly, this change is not safe. This could be controller via --html-whitespace-sensitivity or a new flag.

Solving this issue would first require some research effort into finding out which HTML attributes have values with semantics that allow Prettier to manipulate them. A list of all (most?) HTML attributes can be found at: https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes

Implementation note: some attribute semantics are bound to the HTML element they are part of, e.g. the semantics of name are different between a button element and a meta element; class is a global attribute, so its semantics should be valid for all elements.

Prettier 1.15.2
Playground link

--parser html

Input (class, content, and accept examples):

<div class="a    b
    c d   "></div>
<meta content=" width=device-width  ,
  initial-scale=1.0,
               user-scalable=no"      />
<input type="file" accept=" image/gif  
                               , image/jpeg ">

Output (class, content, and accept examples):

<div
  class="a    b
    c d   "
></div>
<meta
  content=" width=device-width  ,
  initial-scale=1.0,
               user-scalable=no"
/>
<input
  type="file"
  accept=" image/gif  
                               , image/jpeg "
/>

Expected behavior (class, content, and accept examples):

<div class="a b c d"></div>
<meta content="width=device-width, initial-scale=1.0, user-scalable=no" />
<input type="file" accept="image/gif, image/jpeg" />