nim doc --backend:js, nim doc --doccmd:-d:foo, nim r --backend:js, --doccmd:skip + other improvements (#14278)

* `nim doc --backend:js|cpp...`
`nim doc --doccmd:'-d:foo --threads:on'`
`nim r --backend:cpp...` (implies --run --usenimcache)
* --usenimcache works with all targets
* --docCmd:skip now skips compiling snippets; 50X speedup for doc/manual.rst

compiler/cmdlinehelper.nim

@@ -78,7 +78,8 @@ proc loadConfigsAndRunMainCommand*(self: NimProg, cache: IdentCache; conf: Confi
   # XXX This is hacky. We need to find a better way.
   case conf.command
   of "cpp", "compiletocpp":
-    conf.cmd = cmdCompileToCpp
+    conf.backend = backendCpp
+    conf.cmd = cmdCompileToBackend
   else:
     discard