This simplifies code and should be better on memory usage since it is
less fragmented.

Also added `width` and `height` attributes to `Layer`.

Closes #54

Closes #116

Start updating README.md

Refactor & document

Make layer ids optional and make tile layer name optional (like object layers)

Update CI link
Remove things from "Things missing" that were added
Update example to use `parse_file`

Parity with tile/object layers

Remove Map::source, add Tileset::source, change Image::source to PathBuf

Layer id defaults to 0 if missing, which is normally unused.
Tile layer name defaults to an empty String if missing, same as object layers.

Add Github Actions

Support object properties.

Added MIT license file

Closes #66

Fixes #72

There is only one image per Tile OR per Tileset. Added columns field.

Add layer id parsing

Add support for multiline string properties

Fix markdown formatting in CONTRIBUTORS file