Links inside bold/italic text result in invalid HTML

Links inside runs of bold or italic text are rendered as invalid HTML.

For example, the following Markdown:

```markdown
foo **[link](https://example.com)** bar
```

is rendered as:

```html
foo link bar
```

whilst the expected output would be:

```html
foo link bar
```

[More examples here](https://runkit.com/bock/5b0e748a072e510012552c4f).

Two things are unexpected here:

1. The output is invalid because the `` and `` tags are closed in the same order in which they're opened.
2. The `` tag is then re-opened and not closed until the end of the text, causing more of the text to be bold than intended.

```html
foo link bar
```

Inizia riproducendo gli esempi Markdown dell’issue nel parser JavaScript del repository e individua la gestione dei link annidati all’interno di sequenze in grassetto e corsivo. Confronta l’HTML generato con l’output previsto e verifica che gli stessi casi producano tag annidati correttamente, senza modificare la formattazione all’interno dei link.

Scritto dal modello di indicizzazione a partire dal testo della issue.