fix: preserve leading empty lines in code cells (if any) during formatting

[mcanouil]

• Formatting an R code cell with Air stripped all leading empty lines between option directives and code. Python cells were unaffected because their virtual documents include inject lines that push user code off position 0.
• formatBlock now strips leading empty lines from the virtual document and adjusts the line offset accordingly, so formatter edits land at the correct position.
• A normalizeEdit collapses two or more leading empty lines to exactly one; this fires even when no language formatter is active.
• New tests cover: single empty line preserved, multiple collapsed to one, no empty lines unaffected, a hostile formatter proving empty lines are hidden, and normalisation without a formatter — for both R and Python.

Tested interactively on a VSIX installed build with (my formatters: Ruff and Air):

---
title: Leading Empty Lines
format: html
---

```{r}
#| label: one-empty-line

x<-1
```

```{r}
#| label: two-empty-lines


x<-2
```

```{r}
#| label: no-empty-lines
x<-3
```

```{python}
#| label: no-empty-line
x=1;y=2
```

```{python}
#| label: one-empty-line

x=1;y=2
```

```{python}
#| label: two-empty-lines


x=3;y=4
```

Which lead to:

---
title: Leading Empty Lines
format: html
---

```{r}
#| label: one-empty-line

x <- 1
```

```{r}
#| label: two-empty-lines

x <- 2
```

```{r}
#| label: no-empty-lines
x <- 3
```

```{python}
#| label: no-empty-line
x = 1
y = 2
```

```{python}
#| label: one-empty-line

x = 1
y = 2
```

```{python}
#| label: two-empty-lines

x = 3
y = 4
```

Fixes #950

[cscheid]

Do I understand it correctly that the code you added here undoes some of the work that Air does?

[mcanouil]

Yes😅 (but before it does it, here it does not even know there were empty lines at the top of the code cells)

Ruff/Black would have done the same thing if the extension was not injected a comment at the top of the virtual doc.🤷‍♂️

[mcanouil]

To clarify, this PR creates normalisation of the leading empty lines regardless of languages and formatters, this makes things consistent.