Highlighter::getCodeSnippet(): bug fix - too many lines retrieved at start of file

When a code snippet in the middle of code is retrieved, the target snippet length calculation `$length = $linesAfter + $linesBefore + 1;` functions correctly, displaying the target line in the middle, padded by x number of lines before and after.
Given `$linesBefore` and `$linesAfter` both being set to two, this would display as:
```
before
before
target
after
after
```

Similarly, when a code snippet at the end of the code is retrieved, it will work correctly as well, the target line is displayed with x number of lines before. The number of lines _after_ is limited automatically by the length of the `$tokenLines` array.
Given `$linesBefore` and `$linesAfter` both being set to two, this would display as:
```
before
before
target
```

However, when a code snippet at the _start_ of the code is retrieved, the number of lines retrieved was incorrect. The target line would display with `($linesBefore + $linesAfter)` lines _after_ the target line.

Given `$linesBefore` and `$linesAfter` both being set to two, this currently displays as:
```
target
after
after
after
after
```

... while it should be:
```
target
after
after
```

The change in this commit fixes this bug.

Author: jrfnl

src/Highlighter.php

@@ -103,7 +103,13 @@ public function getCodeSnippet($source, $lineNumber, $linesBefore = 2, $linesAft
 
         $offset = $lineNumber - $linesBefore - 1;
         $offset = max($offset, 0);
-        $length = $linesAfter + $linesBefore + 1;
+
+        if ($lineNumber <= $linesBefore) {
+            $length = $lineNumber + $linesAfter;
+        } else {
+            $length = $linesAfter + $linesBefore + 1;
+        }
+
         $tokenLines = array_slice($tokenLines, $offset, $length, $preserveKeys = true);
 
         $lines = $this->colorLines($tokenLines);