Add R Markdown support

[sondalex]

This Pull Request adds support for R Markdown files (extension .rmd and .Rmd). R Markdown is parsed using the existing Markdown parser.

[jamesm131]

Would it be possible to extend this PR to support Quarto files (.qmd), which seems to be the generation of R Markdown (with an extremely similar file format in many cases)? Many of the same people are working on it and it is supported by R Studio.

https://quarto.org/

[sondalex]

Apparently, Quarto enables a few Pandoc extensions. For example Callout Blocks are based on Pandoc fenced_divs' extension.
New options would have to be added to the Markdown Parser to support these extensions.
However, it is possible to add partial .qmd file support using the existing Markdown implementation.

The issue will be that lines with

 ::: {.<word>}

will be checked and potential mistakes reported by vscode-ltex.

I do not have time to add extensions to the parser. However, I can create a new PR at the end of this week/next week based on this PR which will add partial .qmd support.

[valentjn]

Merged via #172.