Don't throw errors on RST tables in Markdown and RstMarkdown modes (#22165)

* Don't throw errors on RST tables in Markdown and RstMarkdown modes

Additions to RST simple tables (#19859) made their parsing more
restrictive, which can introduce problems with of some old
nimforum posts, which have tables with sloppily aligned columns
(like this one:
https://github.com/nim-lang/nimforum/issues/330#issuecomment-1376039966).
Also this strictness contradicts to Markdown style of not getting
in the way (ignoring errors).

So this PR proposes a new strategy of dealing with errors:
* In Markdown and legacy (old default) RstMarkdown we try to
  continue parsing, emitting only warnings
* And only in pure RST mode we throw a error

I expect that this strategy will be applied to more parts of markup code
in the future.

* Don't return anything in `checkColumns`

tests/stdlib/trst.nim

@@ -29,7 +29,9 @@ import os
 import std/[assertions, syncio]
 
 const preferMarkdown = {roPreferMarkdown, roSupportMarkdown, roNimFile, roSandboxDisabled}
+# legacy nimforum / old default mode:
 const preferRst = {roSupportMarkdown, roNimFile, roSandboxDisabled}
+const pureRst = {roNimFile, roSandboxDisabled}
 
 proc toAst(input: string,
             rstOptions: RstParseOptions = preferMarkdown,
@@ -917,10 +919,31 @@ suite "RST tables":
         ======   ======
          Inputs  Output
         ======   ======
-        """.toAst(error=error) == "")
+        """.toAst(rstOptions = pureRst, error = error) == "")
     check(error[] == "input(2, 2) Error: Illformed table: " &
                      "this word crosses table column from the right")
 
+    # In nimforum compatibility mode & Markdown we raise a warning instead:
+    let expected = dedent"""
+      rnTable  colCount=2
+        rnTableRow
+          rnTableDataCell
+            rnLeaf  'Inputs'
+          rnTableDataCell
+            rnLeaf  'Output'
+      """
+    for opt in [preferRst, preferMarkdown]:
+      var warnings = new seq[string]
+
+      check(
+        dedent"""
+          ======   ======
+           Inputs  Output
+          ======   ======
+          """.toAst(rstOptions = opt, warnings = warnings) == expected)
+      check(warnings[] == @[
+        "input(2, 2) Warning: RST style: this word crosses table column from the right"])
+
   test "tables with slightly overflowed cells cause an error (2)":
     var error = new string
     check("" == dedent"""
@@ -929,7 +952,7 @@ suite "RST tables":
       =====  =====  ======
       False  False  False
       =====  =====  ======
-      """.toAst(error=error))
+      """.toAst(rstOptions = pureRst, error = error))
     check(error[] == "input(2, 8) Error: Illformed table: " &
                      "this word crosses table column from the right")
 
@@ -941,7 +964,7 @@ suite "RST tables":
       =====  =====  ======
       False  False  False
       =====  =====  ======
-      """.toAst(error=error))
+      """.toAst(rstOptions = pureRst, error = error))
     check(error[] == "input(2, 7) Error: Illformed table: " &
                      "this word crosses table column from the left")
 
@@ -954,7 +977,7 @@ suite "RST tables":
       =====  ======
       False  False
       =====  =======
-      """.toAst(error=error))
+      """.toAst(rstOptions = pureRst, error = error))
     check(error[] == "input(5, 14) Error: Illformed table: " &
                      "end of table column #2 should end at position 13")
 
@@ -966,7 +989,7 @@ suite "RST tables":
       =====  =======
       False  False
       =====  ======
-      """.toAst(error=error))
+      """.toAst(rstOptions = pureRst, error = error))
     check(error[] == "input(3, 14) Error: Illformed table: " &
                      "end of table column #2 should end at position 13")