feat(import): Add Lua filter for reading unformatted text

Sourced from GH issue comment:
jgm/pandoc#6393 (comment)

lib/plain_reader.lua

@@ -0,0 +1,28 @@
+-- A sample custom reader that just parses text into blankline-separated
+-- paragraphs with space-separated words.
+
+-- For better performance we put these functions in local variables:
+local P, S, V, Ct = lpeg.P, lpeg.S, lpeg.V, lpeg.Ct
+
+local whitespacechar = S(" \t\r\n")
+local wordchar = (1 - whitespacechar)
+local spacechar = S(" \t")
+local newline = P("\r") ^ -1 * P("\n")
+local blanklines = newline * (spacechar ^ 0 * newline) ^ 1
+local endline = newline - blanklines
+
+-- Grammar
+local G = P({
+   "Pandoc",
+   Pandoc = Ct(V("Block") ^ 0) / pandoc.Pandoc,
+   Block = blanklines ^ 0 * V("Para"),
+   Para = Ct(V("Inline") ^ 1) / pandoc.Para,
+   Inline = V("Str") + V("Space") + V("SoftBreak"),
+   Str = wordchar ^ 1 / pandoc.Str,
+   Space = spacechar ^ 1 / pandoc.Space,
+   SoftBreak = endline / pandoc.SoftBreak,
+})
+
+function Reader (input)
+   return lpeg.match(G, tostring(input))
+end