* [HTML] Fix toggle html comments in css/js tags

Fixes sublimehq#2322

In order to enable comment toggling using rules of embedded syntax
definitions it is important to maintain its main scope at the beginning
of first and last line.

The challenge is to also support CSS/JS being wrapped into HTML
comment markers.

Another goal is to keep as many implementation details away from
contexts which may need to be overridden by inheriting syntax
definitions such as ASP/JSP/PHP to keep dependencies low. Some little
changes are required though at this point to accomplish that.

Note: This commit includes the assumption that JavaScript won't contain
      a standalone `-->` on a single line.

* [HTML] Fix toggle comments on first line of script/style content

Fixes #sublimehq/sublime_text/issues/4701

This commit introduces a change to ensure the very first char of the
very first line after a <script> or <style> tag to receive correct
source.js vs. source.css scope in case not html comment is present.

As a result toggling comments gives JS/CSS comments precedence over HTML
comments, while maintaining highlighting of existing HTML comments.

As latter one are present for backward compatibility with no longer supported
browsers of the 90s, it is assumed wrapping JS/CSS into HTML comments is no
longer a common task and thus entering those comments manually
is acceptable once they are needed.

Before this commit hitting `ctrl+/` at

```
<script>
  |
</script>
```

resulted in

```
<script>
  <!-- | -->
</script>
```

Now it results in

```
<script>
  // |
</script>
```

Same for `<style>`.

* [HTML] Simplify CSS/JS embedding

This commit removes one level of abstraction.

The removed separation between  `...-begin` and `...-content` context
turned out to be useless with regards to avoid dependencies between
inherited and inheriting syntax definition.

It's because the inheriting syntax needs to handle those HTML comments
in front of closing tags by itself anyway. So it can do the same for
the opening comment and we can save one useless context switch.

Learned from adding VBScript highlighting in script tags of ASP syntax.

* [HTML] Refine script and style embedding

This comment:

1. directly captures closing html comment punctuation and remaining
   source scopes via escape_captures.

   It avoids `javascript-end` and `style-end` contexts, which used to
   consume those before.

   It avoids inheriting syntaxes to need to struggle with closing tag
   contexts. Once tag names are a bit more generic, we may even drop
   the `script-close-tag` and `style-close-tag` contexts.

2. adds some descriptions and justifications to the patterns used to
   embed and escape from JS/CSS content.

3. refines some test cases.

As a result, an inheriting syntax definition just needs to implement one
context to override existing JS/CSS contexts.