Refactor & document

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

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

update Zstd to 0.9

Reorganize crate

Move parse functions to its respective types, rename `Colour` for consistency with PropertyValue::ColorValue, add documentation to map members, add Map::source, misc improvements