refs #15667 improve invalid indentation errors, report when & where = could be missing (#16397)

* refs #15667 improve invalid indentation errors

* also show line info where = is missing

* add test

* add more tests

tests/parser/t15667.nim

@@ -0,0 +1,58 @@
+discard """
+  cmd: "nim check $options $file"
+  action: "reject"
+  nimout: '''
+t15667.nim(23, 5) Error: invalid indentation, maybe you forgot a '=' at t15667.nim(22, 13) ?
+t15667.nim(28, 5) Error: invalid indentation, maybe you forgot a '=' at t15667.nim(26, 13) ?
+t15667.nim(33, 5) Error: invalid indentation, maybe you forgot a '=' at t15667.nim(31, 25) ?
+t15667.nim(42, 5) Error: invalid indentation, maybe you forgot a '=' at t15667.nim(38, 12) ?
+t15667.nim(56, 5) Error: invalid indentation, maybe you forgot a '=' at t15667.nim(55, 13) ?
+'''
+"""
+
+
+
+
+
+
+
+
+# line 20
+block:
+  proc fn1()
+    discard
+
+block:
+  proc fn2()
+    #
+    discard
+
+block:
+  proc fn3() {.exportc.}
+    #
+    discard
+
+block: # complex example
+  proc asdfasdfsd() {. exportc, 
+      inline
+         .}     # foo
+    #[
+    bar
+    ]#
+    discard
+
+block: # xxx this doesn't work yet (only a bare `invalid indentation` error)
+  proc fn5()
+    ##
+    discard
+
+block: # ditto
+  proc fn6*()
+    ## foo bar
+    runnableExamples: discard
+
+block:
+  proc fn8()
+    runnableExamples:
+      discard
+    discard