feat: sort critical tags and sort opt-in renderPriority

[harlan-zw]

Problem

There are two problems which overlap:

1. There are certain HTML tags that are critical to be rendered first, these include:

• <base> Must be rendered before any tags with href=".."
• <meta charset...> To avoid encoding issues it should come before any text which may not match the default encoding
• <meta http-equiv="content-security-policy"> Needs to come before any scripts

2. Users want to be able to change the order of tags, see Question: how to prioritize some link tags above others #86, Option for ordering in head? font faces loaded before fonts are loaded, results in annoying warning #58

Solution

Sorting the tags is fairly straightforward, for SSR. Which is what the below PR supports.

Things get a bit complicated for client rendering due to how the DOM patching is working. Without re-working the existing patching logic it's not possible to order the tags between Vue node instances. Sorting within the node instance is possible but not as useful to the end user.

Even though non-SSR does not work, the feature is still useful, ordering meta tags is mostly an SEO optimisation in which most people would be using some SSR hybrid rendering strategy for.

[harlan-zw]

FYI I have merged #88 into this branch so there's no conflict once that's merged