[BUGFIX] Add missing imports for @throws annotations (#900)

This is the V8.x backport of #897.

Author: oliverklee

src/Value/CSSFunction.php

@@ -4,6 +4,9 @@
 
 use Sabberworm\CSS\OutputFormat;
 use Sabberworm\CSS\Parsing\ParserState;
+use Sabberworm\CSS\Parsing\SourceException;
+use Sabberworm\CSS\Parsing\UnexpectedEOFException;
+use Sabberworm\CSS\Parsing\UnexpectedTokenException;
 
 /**
  * A `CSSFunction` represents a special kind of value that also contains a function name and where the values are the