[strings] Add utility to test whether a path is absolute

strings.lua

@@ -155,6 +155,28 @@ function strings.splitpath(path)
     return path:match("(.-)([^\\/]-)(%.?[^%.\\/]*)$")
 end
 
+--- Test whether the path is absolute.
+--
+-- @string path a path to be tested
+-- @string[opt] sep separator pattern, '/' for Unix, '\\' for Windows, if none is provided, path is tested  for both
+-- @treturn boolean true if path is absolute, false otherwise
+function strings.isabspath(path, sep)
+    if sep == nil then
+        -- Conservative test, both Unix-style and Windows-style
+        return strings.isabspath(path, '/') or strings.isabspath(path, '\\')
+    elseif sep == '/' then
+        -- Unix-style
+        return #path >= 1 and path:byte(1) == SLASH
+    elseif sep == '\\' then
+        -- Windows-style
+        return
+            (#path >= 1 and path:byte(1) == BACKSLASH) or
+            (#path >= 3 and path:byte(2) == COLON and path:byte(3) == BACKSLASH)
+    else
+        error(("Unsupported separator pattern: %q"):format(sep))
+    end
+end
+
 --- Test whether a string starts with a prefix.
 --
 -- This is an optimized version of: return s:sub(1, #prefix) == prefix.