[12.x] Make Str::is() match multiline strings

[SjorsO]

Currently Str::is() almost always returns false for strings containing a newline, for example:

$multilineString = <<<'STRING'
first
second
third
STRING;

Str::is('*', $multilineString); // currently false
Str::is('first*', $multilineString); // currently false
Str::is("first\n*", $multilineString); // currently false
Str::is("first\nsecond\n*", $multilineString); // currently true but not very practical

This happens because the * wildcard gets changed to the regex pattern .*, but the . doesn't match newlines because the s modifier isn't present.

Str::is() is used by the framework to match fake outputs for the Process facade. Currently you can't properly match a multiline command (as reported in #50158 and fixed with a hack in #50164). This PR fixes this problem properly. For example:

$result = Process::run(<<<'COMMAND'
git clone --depth 1 \
      --single-branch \
      --branch main \
      git://some-url .
'COMMAND');

Process::preventStrayProcesses();

// Currently this pattern will never match and your test will always fail.
// After this PR the pattern works as you'd expect.
Process::fake(['git clone*' => 'faked output']);

After this PR the * pattern always matches everything, so I've added the $pattern === '*' check because it is a minor speed boost.

I've targeted 12.x because this PR is a breaking change. If you rely on Str::is() or str()->is() returning false on multiline strings then you'll have to update your code.