fix undeclared identifier in templates in generics (#24069)

fixes #13979

Fixes templates in generics that use identifiers that aren't defined
yet, giving an early `undeclared identifier` error, by just marking
template bodies as in a mixin context in `semgnrc`.

(cherry picked from commit bf865fa75a)

tests/generics/tnestedtemplate.nim

@@ -0,0 +1,9 @@
+block: # issue #13979
+  var s: seq[int]
+  proc filterScanline[T](input: openArray[T]) =
+    template currPix: untyped = input[i]
+    for i in 0..<input.len:
+      s.add currPix
+  let pix = [1, 2, 3]
+  filterScanline(pix)
+  doAssert s == @[1, 2, 3]