RST: improve simple tables (#19859)

* RST: improve simple tables

* nim 1.0 gotchas

* Still allow legacy boundaries like `----`

compiler/docgen.nim

@@ -232,6 +232,7 @@ template declareClosures =
     of meExpected: k = errXExpected
     of meGridTableNotImplemented: k = errRstGridTableNotImplemented
     of meMarkdownIllformedTable: k = errRstMarkdownIllformedTable
+    of meIllformedTable: k = errRstIllformedTable
     of meNewSectionExpected: k = errRstNewSectionExpected
     of meGeneralParseError: k = errRstGeneralParseError
     of meInvalidDirective: k = errRstInvalidDirectiveX