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

lib/std/compilesettings.nim

@@ -29,6 +29,8 @@ type
     linkOptions,      ## additional options passed to the linker
     compileOptions,   ## additional options passed to the C/C++ compiler
     ccompilerPath     ## the path to the C/C++ compiler
+    backend           ## the backend (eg: c|cpp|objc|js); both `nim doc --backend:js`
+                      ## and `nim js` would imply backend=js
 
   MultipleValueSetting* {.pure.} = enum ## \
                       ## settings resulting in a seq of string values