feat(compiler): recover parser from incomplete open tag

[znck]

<div>
  <div |
</div>

and

<div |
<div></div>

are the most common error state when editing <template>. This PR processes these partial/incomplete tags as self-closing elements. Parsing these tags as elements is important as node type is used in VueDX to provide completion.

[skirtles-code]

@znck. Do you think this is still worth pursuing?

The file compiler-core/src/parse.ts no longer exists since the parser was rewritten, so it isn't clear to me how this change would be applied to the latest code on main.