Improve tests embebbing

[Vladyslav-Kuksiuk]

This PR improves pattern-based embedding by adding explicit multi-line matching for start, end and line patterns. Instead of requiring a single source line to identify a fragment boundary, patterns can now use \n to match consecutive source lines.

For example:

<embed-code
  file="$test/JunitTest.java"
  start="Test \n adds two values"
  end="assertEquals(2, value); \n }"
/>
```java
@Test
@DisplayName("adds two values")
void addsTwoValues() {
    int value = 1 + 1;

    assertEquals(2, value);
}
```

Each line in a multi-line pattern follows the same glob rules as regular one-line patterns, including ^ and $ for exact matching.

Spaces around \n are ignored.

Additional change: added possibility to write quote characters in patterns as \" instead of the XML entity ".

Resolves SpineEventEngine/embed-code-jekyll#22.