[Console] handles multi-byte characters in autocomplete

[jls-esokia]

fixes #29966

Q	A
Branch?	3.4
Bug fix?	yes
New feature?	no
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets	#29966
License	MIT
Doc PR	-

I used the mb_ord to detect whether the amount of bytes read is valid before proceeding. I limit the number of bytes read to 4 before giving up because characters can use at most 4 bytes.
The test passes with or without the fix though.

[nicolas-grekas]

Using mb_ord() requires PHP 7.2, but the lowest supported version for branch 3.4 is PHP 5.5.
We could use the 7.2 polyfill to make this work, but I don't think the polyfill implements the case where it should return false on invalid characters.
That'd be great if someone could have a look at the polyfill to fix it.

For this PR, we could use that map:
$lengthMask = array("\xC0" => 2, "\xD0" => 2, "\xE0" => 3, "\xF0" => 4);
Then, for a byte $b, knowing the length of the utf8 character it starts is done with:
$length = $b < "\x80" ? 1 : $lengthMask[$b & "\xF0"];

[jls-esokia]

My bad, I saw mb_ord was listed in the polyfill-mbstring library which is a dependency of the console component. Ok made the necessary correction.

[OskarStark]

Suggested change

	$this->assertSame($character, $dialog->ask($this->createStreamableInputInterfaceMock($inputStream), $this->createOutputInterface(), $question));
	$this->assertSame(
	$character,
	$dialog->ask($this->createStreamableInputInterfaceMock($inputStream), $this->createOutputInterface(), $question)
	);

[nicolas-grekas]

we usually prefer long lines so not sure about that :)

[nicolas-grekas]

Thank you @jls-esokia.