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

tests/nimdoc/m13129.nim

@@ -0,0 +1,36 @@
+when defined(cpp):
+  {.push header: "<vector>".}
+  type
+    Vector[T] {.importcpp: "std::vector".} = object
+elif defined(js):
+  proc endsWith*(s, suffix: cstring): bool {.noSideEffect,importjs: "#.endsWith(#)".}
+elif defined(c):
+  proc c_printf*(frmt: cstring): cint {.
+    importc: "printf", header: "<stdio.h>", varargs, discardable.}
+
+proc main*() =
+  runnableExamples:
+    import std/compilesettings
+    doAssert not defined(m13129Foo1)
+    doAssert defined(m13129Foo2)
+    doAssert not defined(nimdoc)
+    echo "ok2: backend: " & querySetting(backend)
+    # echo defined(c), defined(js), 
+
+import std/compilesettings
+when defined nimdoc:
+  # import std/compilesettings
+  static:
+    doAssert defined(m13129Foo1)
+    doAssert not defined(m13129Foo2)
+    echo "ok1:" & querySetting(backend)
+
+when isMainModule:
+  when not defined(js):
+    import std/os
+    let cache = querySetting(nimcacheDir)
+    doAssert cache.len > 0
+    let app = getAppFilename()
+    doAssert app.isRelativeTo(cache)
+    doAssert querySetting(projectFull) == currentSourcePath
+    echo "ok3"

tests/nimdoc/readme.md

@@ -0,0 +1,2 @@
+the html validation is tested by nimdoc/tester.nim
+the runnableExamples + nim doc logic (across backend) is tested here