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/semexprs.nim

@@ -660,7 +660,7 @@ proc hasUnresolvedArgs(c: PContext, n: PNode): bool =
     return false
 
 proc newHiddenAddrTaken(c: PContext, n: PNode): PNode =
-  if n.kind == nkHiddenDeref and not (c.config.cmd == cmdCompileToCpp or
+  if n.kind == nkHiddenDeref and not (c.config.backend == backendCpp or
                                       sfCompileToCpp in c.module.flags):
     checkSonsLen(n, 1, c.config)
     result = n[0]