[TwigComponent] Fix twig:blockquote is not a block

[smnandre]

Q	A
Bug fix?	yes
New feature?	no
Deprecations?	no
Issues	-
License	MIT

A component whose name starts with block breaks the block it sits in.

Reproducer

<twig:Card>
    <twig:block name="footer">
        <twig:blockquote cite="Matt">He ho!</twig:blockquote>
    </twig:block>
</twig:Card>

Twig\Error\SyntaxError: Expected closing tag "</twig:Card>" not found at line 5.

The error points at Card, which is correctly closed, and at the last line of the
template. Nothing mentions blockquote, so there is little to go on.

Same failure with <twig:blockquote />, <twig:block-title />, <twig:blocked />, and
inside a traditional {% block %} … {% endblock %}.

Not affected: <twig:Blockquote /> (the comparison is case-sensitive), and any of these
outside a block.

Cause

TwigPreLexer::consumeUntilEndBlock() tracks block nesting with a fixed-length prefix
comparison:

if (!$inComment && '<twig:block' === substr($this->input, $this->position, 11)) {
    ++$depth;
}

'<twig:block' is also the first 11 characters of <twig:blockquote, so the depth is
incremented for a component that is not a block. It is never decremented back: the
closing comparison is '</twig:block>', which ends with > and therefore does not
match </twig:blockquote>. The enclosing <twig:block> never terminates, scanning runs
to the end of the template, and the outer component is reported as unclosed.

Fix

Anchor the match and require that block is not followed by another component-name
character, using the same character class as consumeComponentName():

if (!$inComment && preg_match('/\G<twig:block(?![A-Za-z0-9_:@\-.])/', $this->input, $matches, 0, $this->position)) {
    ++$depth;
}

The closing comparison needs no change.