Implement codegenDecl for js (#6851)

tests/js/tcodegendeclproc.nim

@@ -0,0 +1,11 @@
+discard """
+  output: '''
+-1
+8
+'''
+  ccodecheck: "'console.log(-1); function fac_' \\d+ '(n_' \\d+ ')'"
+"""
+proc fac(n: int): int {.codegenDecl: "console.log(-1); function $2($3)".} =
+  return n
+
+echo fac(8)

tests/js/tcodegendeclvar.nim

@@ -0,0 +1,10 @@
+discard """
+  output: '''
+-1
+2
+'''
+  ccodecheck: "'console.log(-1); var v_' \\d+ ' = [2]'"
+"""
+
+var v {.codegenDecl: "console.log(-1); var $2".} = 2
+echo v