pragma for sfCallsite instead of name check + better semantics, test (#20464)

* pragma for sfCallsite instead of name check at every template definition

Not documented because it seems to be for internal use?

Should also make it possible to make comparisons and setops imports, but this doesn't have to be done.

I can reuse a name like `cursor` for the pragma as well, added a new name just to be safe.

* make sfCallsite recursive, add tests

tests/template/tcallsitelineinfo.nim

@@ -0,0 +1,17 @@
+discard """
+  nimout: '''
+tcallsitelineinfo.nim(17, 4) Warning: abc [User]
+'''
+  exitcode: 1
+  outputsub: '''
+tcallsitelineinfo.nim(17) tcallsitelineinfo
+Error: unhandled exception: def [ValueError]
+'''
+"""
+
+template foo(): untyped {.callsite.} =
+  {.warning: "abc".}
+  raise newException(ValueError, "def")
+  echo "hello"
+
+foo()

tests/template/tcallsitelineinfo2.nim

@@ -0,0 +1,20 @@
+discard """
+  nimout: '''
+tcallsitelineinfo2.nim(18, 1) Warning: abc [User]
+tcallsitelineinfo2.nim(19, 12) Warning: def [User]
+'''
+  exitcode: 1
+  outputsub: '''
+tcallsitelineinfo2.nim(20) tcallsitelineinfo2
+Error: unhandled exception: ghi [ValueError]
+'''
+"""
+
+template foo(a: untyped): untyped {.callsite.} =
+  {.warning: "abc".}
+  a
+  echo "hello"
+
+foo: # with `{.line.}:`, the following do not keep their line information:
+  {.warning: "def".}
+  raise newException(ValueError, "ghi")