Make static blocks introduce their own scope

Treat the static block as a normal block, don't leak any identifier in
the outer scope.

Fixes #5958

compiler/semstmts.nim

@@ -1772,7 +1772,9 @@ proc semStaticStmt(c: PContext, n: PNode): PNode =
   #echo "semStaticStmt"
   #writeStackTrace()
   inc c.inStaticContext
+  openScope(c)
   let a = semStmt(c, n.sons[0])
+  closeScope(c)
   dec c.inStaticContext
   n.sons[0] = a
   evalStaticStmt(c.module, c.graph, a, c.p.owner)

tests/global/t5958.nim

@@ -0,0 +1,9 @@
+discard """
+  line: 9
+  errormsg: "undeclared identifier: 'a'"
+"""
+
+static:
+  var a = 1
+
+echo a