Improve simple_tag / simple_block_tag error message when as is used without a variable name

[Shima Fallah]

When using Library.simple_tag() or Library.simple_block_tag(), templates that end with as but omit the target variable currently raise a generic argument error. This is valid behavior (it fails), but the message is unclear for template authors.

Examples
{% load custom %}{% one_param 37 as %}
{% load custom %}{% div as %}content{% enddiv %}
Current behavior
A generic TemplateSyntaxError is raised from argument parsing, which does not clearly explain that the variable name after as is missing.

Expected behavior
Raise a clear, direct error, e.g.:

'one_param' tag requires a variable name after 'as'
'div' tag requires a variable name after 'as'
What this change does

Adds an explicit check in both compile paths (simple_tag and simple_block_tag) for trailing as without a following variable.
Raises a specific TemplateSyntaxError message before normal argument parsing.
Tests
Adds 4 regression tests covering invalid trailing as usage:

simple tag without args: {% no_params as %}
simple tag with args: {% one_param 37 as %}
simple block tag without args: {% div as %}...{% enddiv %}
simple block tag with args: {% one_param_block 37 as %}...{% endone_param_block %}

These tests verify the new explicit error messages.

[Shima Fallah]

I have submitted a pull request here: ​https://github.com/django/django/pull/21134
I've fixed the code styling issues (line length) in my branch. Ready to open a new PR as soon as the ticket is accepted.