ark depends on non-exported R_Is_Running; breaks compilation on Linux

[jmcphers]

Compiling ark on Linux results in the following error:

/usr/bin/ld: /home/jmcphers/git/positron/extensions/positron-r/amalthea/crates/ark/src/lsp/handler.rs:
(.text._ZN86_$LT$ark..lsp..handler..Lsp$u20$as$u20$amalthea..language..lsp_handler..LspHandler$GT$5start28_$u7b$$u7b$closure$u7d$$u7d$17h189ba1405fc549d7E+0x28): 
undefined reference to `R_Is_Running'

This is probably because R_Is_Running is not exposed in libR.so on Linux; that is, it is just extern0 not LibExtern in Defn.h:

https://github.com/wch/r-source/blob/87f8be154f4f8a72683a66cf0af62b6b5017b084/src/include/Defn.h#L1398

We use it here:

https://github.com/rstudio/positron/blob/adf3167ed747ca0db1d16a133ad01fa1da5825c2/extensions/positron-r/amalthea/crates/ark/src/lsp/handler.rs#L48-L52

Not sure why this doesn't break on macOS (maybe all symbols are exported in the .dylib?), but we should fix it for Linux so we can get CI running there.

[kevinushey]

Thanks; I'm going to take a look -- I'm hoping to coordinate between the shell + LSP threads to make sure the LSP only starts after the shell knows the kernel is ready.