Add testcase for #10465 (#14943)

* add debug format string

* remove try except

* add changelog

* add docs and more tests

* Update lib/pure/strformat.nim

Co-authored-by: Juan Carlos <juancarlospaco@gmail.com>

* minor

* add testcase

Co-authored-by: Juan Carlos <juancarlospaco@gmail.com>

tests/constr/tconexpr.nim

@@ -0,0 +1,43 @@
+discard """
+  nimout: '''
+Fibonacci sequence: 0, 1, 1, 2, 3
+Sequence continues: 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610
+'''
+"""
+
+
+import strformat
+
+var fib_n {.compileTime.}: int
+var fib_prev {.compileTime.}: int
+var fib_prev_prev {.compileTime.}: int
+
+proc next_fib(): int {.compileTime.} =
+  let fib = if fib_n < 2:
+    fib_n
+  else:
+    fib_prev_prev + fib_prev
+  inc(fib_n)
+  fib_prev_prev = fib_prev
+  fib_prev = fib
+  fib
+
+const f0 = next_fib()
+const f1 = next_fib()
+const f2 = next_fib()
+const f3 = next_fib()
+const f4 = next_fib()
+
+static:
+  echo fmt"Fibonacci sequence: {f0}, {f1}, {f2}, {f3}, {f4}"
+
+const fib_continues = block:
+  var result = fmt"Sequence continues: "
+  for i in 0..10:
+    if i > 0:
+      add(result, ", ")
+    add(result, $next_fib())
+  result
+
+static:
+  echo fib_continues