style fix: change 'JS' to 'js' to make it consistent (#13168)

lib/system/arithmetics.nim

@@ -105,7 +105,7 @@ when defined(nimNoZeroExtendMagic):
     ## **Deprecated since version 0.19.9**: Use unsigned integers instead.
     cast[int32](x)
 
-elif not defined(JS):
+elif not defined(js):
   proc ze*(x: int8): int {.magic: "Ze8ToI", noSideEffect, deprecated.}
     ## zero extends a smaller integer type to ``int``. This treats `x` as
     ## unsigned.

lib/system/comparisons.nim

@@ -283,7 +283,7 @@ proc `==`*[T](x, y: seq[T]): bool {.noSideEffect.} =
       if x.len == 0 and y.len == 0:
         return true
   else:
-    when not defined(JS):
+    when not defined(js):
       proc seqToPtr[T](x: seq[T]): pointer {.inline, noSideEffect.} =
         when defined(nimSeqsV2):
           result = cast[NimSeqV2[T]](x).p

lib/system/gc_interface.nim

@@ -13,7 +13,7 @@ when hasAlloc:
       gcOptimizeTime,    ## optimize for speed
       gcOptimizeSpace    ## optimize for memory footprint
 
-when hasAlloc and not defined(JS) and not usesDestructors:
+when hasAlloc and not defined(js) and not usesDestructors:
   proc GC_disable*() {.rtl, inl, benign.}
     ## Disables the GC. If called `n` times, `n` calls to `GC_enable`
     ## are needed to reactivate the GC.
@@ -58,7 +58,7 @@ when hasAlloc and not defined(JS) and not usesDestructors:
     ## Expands operating GC stack range to `theStackBottom`. Does nothing
       ## if current stack bottom is already lower than `theStackBottom`.
 
-when hasAlloc and defined(JS):
+when hasAlloc and defined(js):
   template GC_disable* =
     {.warning: "GC_disable is a no-op in JavaScript".}
 

lib/system/memalloc.nim

@@ -229,7 +229,7 @@ when hasAlloc:
     ## Returns the number of bytes that are owned by the process.
 
 
-when defined(JS):
+when defined(js):
   # Stubs:
   proc getOccupiedMem(): int = return -1
   proc getFreeMem(): int = return -1