Allow xmlns= on <template> elements

[LukeShu]

• Vue components are based on HTML5 custom elements. However, HTML5
  custom elements are required to be in the HTML namespace. This is not a
  requirement shared by Vue components. Vue itself doesn't care about the
  namespace of the elements that it is rendering (which will be determined
  at runtime based on the namespace of the parent element where it's being
  inserted). Therefore, it is desirable to communicate to linters and
  other tools what the intended namespace of a top-level <template> in an
  SFC is.

  See: https://html.spec.whatwg.org/multipage/custom-elements.html#look-up-a-custom-element-definition

• Vue slots are based on HTML5 slots. However, because of the above
  limitation that HTML5 custom elements are always in the HTML5 namespace,
  the slots contained in them are also always in the HTML5 namespace.
  Because of Vue's flexibility on this, a Vue <slot> may appear in any
  namespace; content being rendered into a slot may end up in a different
  namespace than the namespace where it was parsed. Therefore, it is
  desirable to communicate to linters and other tools what the indented
  namespace is of content that will be rendered in a slot.

From these two observations, it would be useful to set xmlns= on:

1. top-level <template> elements, and
2. elements appearing in a slot.

The first is easy, but #2 has some concerns:

• It's relatively difficult to determine when we're appearing in a slot.
  We could try checking for the slot= attribute, but that would miss cases
  where we use the default slot.
• Because the xmlns= attribute on the resulting element would be invalid
  in HTML5 (in the cases where this is useful, anyway), we can't set it on
  normal elements anyway.

So, mostly because of the second reason, let's just say that to inform
linters about a change in namespace because of a slot, the contents must
be wrapped in a <template> to safely set xmlns= in a place that will be
ignored at runtime. So now we've revised that to allowing xmlns= on:

1. top-level <template> elements, and
2. <template> elements appearing in a slot.

Let's just make things simple and allow it for all <template> elements.
This means we'll be slightly too lax, and allow it on <template v-if="...">
constructs, but I'm OK with that.

As for which namespaces to consider valid, let's limit that to HTML, SVG,
and MathML, as those are the namespaces that are allowed in an HTML5
document.

[codecov-io]

Codecov Report

Merging #27 into master will increase coverage by <.01%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master      #27      +/-   ##
==========================================
+ Coverage   85.54%   85.55%   +<.01%     
==========================================
  Files          32       32              
  Lines        1847     1855       +8     
  Branches      476      481       +5     
==========================================
+ Hits         1580     1587       +7     
  Misses        181      181              
- Partials       86       87       +1

Impacted Files	Coverage Δ
src/html/parser.ts	96.17% <88.88%> (-0.47%)	⬇️

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[mysticatea]

Sounds reasonable to me. Thank you for contributing!