Fix Vue pug rendering by specifying HTML5 by default

[Jengamon]

↪️ Pull Request

For the pug template engine for Vue, make HTML5 the default. This will fix #7756.
See discussion at #8050 for considerations about this change.

💻 Examples

<template lang="pug">

.wrapper

	p(v-if="false") foo
	p(v-else) bug

</template>

should now render properly without any workarounds.

🚨 Test instructions

I will add the above example as an integration test.

✔️ PR Todo

• Added/updated unit tests for this change
• Filled out test instructions (In case there aren't any unit tests)
• Included links to related issues/PRs

[Jengamon]

Does this need a whole integration test?

[101arrowz]

LGTM