Refactor folder structure after the scheme of Jinja2 package.

This commit...

1. introduces common `meta.template.twig` applied scope accross Text/HTML/JS/CSS
2. adjusts snippet selectors
3. adds some helper key bindings
4. applies syntax specific toggle comment rules to apply Twig comments
   by default but retain original syntax's block comment as variant 2.
5. renames contexts to use hyphens as subword-separators
6. reorders contexts following lessons learned from Jinja and other template
   syntaxes
7. apply `meta.embedded` to tags and `source.twig` to their content.
8. fix various smaller highlighting bugs

A formatting change and removal of some default operator/operand keys.

This commit enables to test embedded CSS, JS, JSON code in Twig templates.

Without it, all test assertions cause illegal syntax highlighting
in script or style tags.

Note: It drops some tests for leading `{{` and `{%` due to assertion punctuation
now requiring 2 chars, but those are most often redundant anyway.

- reorder style tag context
- add some backward compatibility fixes

Supported by Twig 3.15+

Adds snippets for individual tags so typing a tag name and hitting tab key
expands to an individual tag enclosed in `{% ... %}`:

   if| =>  {% if | %}
   embed| =>  {% embed | %}

Triggers of existing block snippets are prefixed with `b` to avoid ambiguities
with tag-snippets. Otherwise it would be unclear whether blocks or tags are
expanded and/or completion panel was displayed requiring arrow key navigation.

   bif|

   {% if conditon %}
      |
   {% endif %}

Reorganize snippets in individual sub-directories in case someone wants to
disable individual ones based on type.

Twig tags within quoted strings can contain same sort of quotes.

This requires another strategy of handling embedded code blocks
especially in inline event and style attributes.

Solution is borrowed from PHP.

Instead of adding rules to pop on quotes to each context,
this commit just replaces relevant string contexts of JS/CSS
to make sure to pop contexts properly.

This reduces compiled syntax cache from 820kB to 610kB.

This commit fixes Twig statement tags and comments being scoped
meta.attribute-with-value by accident.

Add some info about changed snippets.

Snippets are basically aligned with Jinja2.