Preserve information about leading whitespace in paragraphs

When using DocumentParser with no enabled block types, the Text nodes do not include whitespaces at the beginning of a line.

For example, when we use a parser with no enabled block types and the input is:

- text 1
  - text 2

the expected result is a document containing two Text nodes:

• Text("- text 1")
• Text(" - text 2")

but the second Text is "- text 2" (without preceding whitespaces)

To better illustrate here is a sample test that fails:

public class ParserTest {
    
    ...
    
    @Test
    public void noBlockTypes() {
        String given = "- text 1\n  - text 2";
        Parser parser = Parser.builder().enabledBlockTypes(Collections.<Class<? extends Block>>emptySet()).build();
        Node document = parser.parse(given);

        Node child = document.getFirstChild();
        assertThat(child, instanceOf(Paragraph.class));

        child = child.getFirstChild();
        assertThat(child, instanceOf(Text.class));
        assertEquals("- text 1", ((Text) child).getLiteral());

        child = child.getNext();
        assertThat(child, instanceOf(SoftLineBreak.class));

        child = child.getNext();
        assertThat(child, instanceOf(Text.class));
        assertEquals("  - text 2", ((Text) child).getLiteral());
    }
}

Inizia dallo scenario fallito ParserTest.noBlockTypes e ispeziona Parser.builder() con tutti i tipi di blocco disabilitati, quindi segui il modo in cui le righe di input diventano nodi Text. Esegui il test con l'input di esempio e conferma che il secondo nodo Text conserva i suoi due spazi iniziali.