fix default R file extension

[ThomasSoeiro]

VSCode recently fixed R file extension to defaults to .R rather than .r: microsoft/vscode#284740 and microsoft/vscode#289658

However this does not work when the vscode-R extension is loaded. This seems to be overwritten by

vscode-R-syntax/package.json

Lines 35 to 42 in d5fbd7a

	"extensions": [
	".r",
	".R",
	".r",
	".rhistory",
	".rprofile",
	".rt"
	],

[ThomasSoeiro]

By the way, we should also get .Rd and .Rmd by default (though I guess its defined in vscode-R extension).

Happy to provide the PRs if it helps!