Avoid .R file lock on windows

[ManuelHentschel]

This PR avoids a file lock issue (observed on windows, probably not an issue on linux): Sourcing .R file using R -f xxx.R locks the file and as a consequence it's impossible to reinstall this extension while it is running. This is particularly annoying when working on the extension itself, since it contains R code and is therefore activated automatically.

This problem is solved by sourcing .R files using R -e source('xxx.r'), which does not appear to lock the file.

[renkun-ken]

Not sure if it is a good idea to use source() instead of direct supplying a file. Changing the content of an R file while it is running by Rscript might cause the R session to fail sometimes.

[ManuelHentschel]

Not sure if it is a good idea to use source() instead of direct supplying a file. Changing the content of an R file while it is running by Rscript might cause the R session to fail sometimes.

This problem would be solved by using source(), since source first parses the entire file and then starts executing. Therefore, background processes like the languageserver and helpserver do not depend on the original file at all, once they start running. IMO this is the ideal behavior. In contrast, on windows Rscript locks the file, making it impossible to quickly reinstall the extension.

On Linux/macos, I'm not sure how exactly this is handled by Rscript. Apparently it is possible to write a script that deletes itself (and does something afterwards) and run it with Rscript, but I'm not sure what the limits to this are (i.e. whether Rscript always reads the entire file first, or I just got lucky and my test script was short enough to be read at once).

[renkun-ken]

LGTM