Add new toLatex() method for nicer printing in PDFs

[andrewheiss]

Base R's sessionInfo() has a toLatex() method for pretty printing in LaTeX PDFs:

```{r}
#| results: asis
toLatex(sessionInfo())
```

This PR fixes #82 and adds a similar method for session_info():

```{r}
#| results: asis
toLatex(sessioninfo::session_info())
```

It supports all the different potential info sections that session_info() can generate:

```{r}
#| results: asis
toLatex(sessioninfo::session_info(info = "all"))
```

[tanho63]

@andrewheiss I think this should close #82 and not #88?

[andrewheiss]

Ah yep, I accidentally hit the wrong issue in the popup menu (#82 is in the original commit)! It's #82 !