toJson, jsonTo, json (de)serialization for custom types; remove dependency on strtabs thanks to a hooking mechanism (#14563)

* json custom serialization; application for strtabs
* serialize using nesting
* make toJson more feature complete
* add since
* Revert "Improve JSON serialisation of strtabs (#14549)"

This reverts commit 7cb4ef26ad.

* better approach via mixin
* toJson, jsonTo
* fix test
* address comments
* move to jsonutils
* doc
* cleanups
* also test for js
* also test for vm

tests/stdlib/tjsonutils.nim

@@ -0,0 +1,44 @@
+discard """
+  targets: "c cpp js"
+"""
+
+import std/jsonutils
+import std/json
+
+proc testRoundtrip[T](t: T, expected: string) =
+  let j = t.toJson
+  doAssert $j == expected, $j
+  doAssert j.jsonTo(T).toJson == j
+
+import tables
+import strtabs
+
+template fn() = 
+  block: # toJson, jsonTo
+    type Foo = distinct float
+    testRoundtrip('x', """120""")
+    when not defined(js):
+      testRoundtrip(cast[pointer](12345)): """12345"""
+
+    # causes workaround in `fromJson` potentially related to
+    # https://github.com/nim-lang/Nim/issues/12282
+    testRoundtrip(Foo(1.5)): """1.5"""
+
+  block:
+    testRoundtrip({"z": "Z", "y": "Y"}.toOrderedTable): """{"z":"Z","y":"Y"}"""
+    when not defined(js): # pending https://github.com/nim-lang/Nim/issues/14574
+      testRoundtrip({"z": (f1: 'f'), }.toTable): """{"z":{"f1":102}}"""
+
+  block:
+    testRoundtrip({"name": "John", "city": "Monaco"}.newStringTable): """{"mode":"modeCaseSensitive","table":{"city":"Monaco","name":"John"}}"""
+
+  block: # complex example
+    let t = {"z": "Z", "y": "Y"}.newStringTable
+    type A = ref object
+      a1: string
+    let a = (1.1, "fo", 'x', @[10,11], [true, false], [t,newStringTable()], [0'i8,3'i8], -4'i16, (foo: 0.5'f32, bar: A(a1: "abc"), bar2: A.default))
+    testRoundtrip(a):
+      """[1.1,"fo",120,[10,11],[true,false],[{"mode":"modeCaseSensitive","table":{"y":"Y","z":"Z"}},{"mode":"modeCaseSensitive","table":{}}],[0,3],-4,{"foo":0.5,"bar":{"a1":"abc"},"bar2":null}]"""
+
+static: fn()
+fn()