fix js stacktraces, unify all file,line,col formatting into a single function (#14230)

* fix https://github.com/timotheecour/Nim/issues/135 ; unify all file,line,col formatting into a single function

lib/std/private/miscdollars.nim

@@ -0,0 +1,15 @@
+template toLocation*(result: var string, file: string | cstring, line: int, col: int) =
+  ## avoids spurious allocations
+  # Hopefully this can be re-used everywhere so that if a user needs to customize,
+  # it can be done in a single place.
+  result.add file
+  if line > 0:
+    result.add "("
+    # simplify this after moving moving `include strmantle` above import assertions`
+    when declared(addInt): result.addInt line
+    else: result.add $line
+    if col > 0:
+      result.add ", "
+      when declared(addInt): result.addInt col
+      else: result.add $col
+    result.add ")"