refactor: move reverse_text to strings.c as it's a string operation

Also add tests for reverse_text.

Co-authored-by: Kalle Ranki <kalle.ranki@gmail.com>

test/unit/strings_spec.lua

@@ -150,3 +150,38 @@ describe('strcase_save()' , function()
     eq("a", strcase_save("\xc1\x81", false))
   end)
 end)
+
+describe("reverse_text", function()
+  local reverse_text = function(str)
+    return helpers.internalize(strings.reverse_text(to_cstr(str)))
+  end
+
+  itp("handles empty string", function()
+    eq("", reverse_text(""))
+  end)
+
+  itp("handles simple cases", function()
+    eq("a", reverse_text("a"))
+    eq("ba", reverse_text("ab"))
+  end)
+
+  itp("handles multibyte characters", function()
+    eq("bα", reverse_text("αb"))
+    eq("Yötön yö", reverse_text("öy nötöY"))
+  end)
+
+  itp("handles combining chars", function()
+    local utf8_COMBINING_RING_ABOVE = "\204\138"
+    local utf8_COMBINING_RING_BELOW = "\204\165"
+    eq("bba" .. utf8_COMBINING_RING_ABOVE .. utf8_COMBINING_RING_BELOW .. "aa",
+       reverse_text("aaa" .. utf8_COMBINING_RING_ABOVE .. utf8_COMBINING_RING_BELOW .. "bb"))
+  end)
+
+  itp("treats invalid utf as separate characters", function()
+    eq("\192ba", reverse_text("ab\192"))
+  end)
+
+  itp("treats an incomplete utf continuation sequence as valid", function()
+    eq("\194ba", reverse_text("ab\194"))
+  end)
+end)