Add new 'r.rmarkdownRender' command

[juliasilge]

Intent

Addresses #1421 by adding a command to do rmarkdown::render() in a fresh R session (i.e. R terminal)

Approach

This PR adds a command that is similar to 'r.sourceCurrentFile' but checks to see if rmarkdown is installed and runs it in a new terminal.

QA Notes

We should check how this command works both with/without the Quarto extension installed. It contributes both a command to the command palette and to the "editor/title/run" bar, but only for .Rmd and not .qmd. The 'r.sourceCurrentFile' command should all still work as expected.

You can make a file saved as .Rmd such as this:

---
title: "A wonderful R Markdown document"
output:
  html_document:
    math_method: katex
---

```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = TRUE)
```

## R Markdown

This is an R Markdown document. Markdown is a simple formatting syntax for authoring HTML, PDF, and MS Word documents. For more details on using R Markdown see <http://rmarkdown.rstudio.com>.

Here is some math:

$1 + 1$

You can embed an R code chunk like this:

```{r cars}
summary(cars)
```

## Including Plots

You can also embed plots, for example:

```{r pressure, echo=FALSE}
plot(pressure)
```

Note that the `echo = FALSE` parameter was added to the code chunk to prevent printing of the R code that generated the plot.

[juliasilge]

This PR does not provide support for previewing any R Markdown, only doing rmarkdown::render().

[juliasilge]

If the Quarto extension is installed, you see this in the "editor/title/run" spot:

If the Quarto extension is not installed, you will see a play button that only has the R Markdown option.

[juliasilge]

With the Quarto extension installed, you see these options in the command palette: