Use "sidebars" for markdown tables.

Adds `|` columns on each side of markdown tables to avoid trailing
whitespace problems in rendered documents. This is still valid markdown.

This does not change the parsing of markdown tables, which can still
be written without the `|` columns on each side.

(cherry picked from commit 14a4fed)
ref #15530

base/markdown/GitHub/table.jl

@@ -97,20 +97,22 @@ function padcells!(rows, align; len = length, min = 0)
 end
 
 _dash(width, align) =
-    align == :l ? ":" * "-"^(width-1) :
-    align == :r ? "-"^(width-1) * ":" :
-    align == :c ? ":" * "-"^(width-2) * ":" :
+    align == :l ? ":" * "-"^width * " " :
+    align == :r ? " " * "-"^width * ":" :
+    align == :c ? ":" * "-"^width * ":" :
     throw(ArgumentError("Invalid alignment $align"))
 
 function plain(io::IO, md::Table)
     cells = mapmap(plaininline, md.rows)
     padcells!(cells, md.align, len = length, min = 3)
     for i = 1:length(cells)
+        print(io, "| ")
         print_joined(io, cells[i], " | ")
-        println(io)
+        println(io, " |")
         if i == 1
-            print_joined(io, [_dash(length(cells[i][j]), md.align[j]) for j = 1:length(cells[1])], " | ")
-            println(io)
+            print(io, "|")
+            print_joined(io, [_dash(length(cells[i][j]), md.align[j]) for j = 1:length(cells[1])], "|")
+            println(io, "|")
         end
     end
 end

test/markdown.jl

@@ -278,12 +278,32 @@ no|table
 no error
 """ == MD([Paragraph(Any["no|table no error"])])
 
-t = """a   |   b
-:-- | --:
-1   |   2
-"""
-@test plain(Markdown.parse(t)) == t
+let t = """a   |   b
+    :-- | --:
+    1   |   2
+    """
+    @test Markdown.parse(t) == MD(Table(Any[Any["a", "b"], Any["1", "2"]], [:l, :r]))
+end
 
+let text =
+    """
+    | a   |   b |
+    |:--- | ---:|
+    | 1   |   2 |
+    """,
+    table = Markdown.parse(text)
+    @test text == Markdown.plain(table)
+end
+let text =
+    """
+    | Markdown | Table |  Test |
+    |:-------- |:-----:| -----:|
+    | foo      | `bar` | *baz* |
+    | `bar`    |  baz  | *foo* |
+    """,
+    table = Markdown.parse(text)
+    @test text == Markdown.plain(table)
+end
 
 # LaTeX extension
 latex_doc = md"""