Missing "file" custom property type support.

[MaulingMonkey]

https://doc.mapeditor.org/en/stable/manual/custom-properties/#adding-properties lists "file" properties, which don't appear to be supported in PropertyValue::new:

https://github.com/mattyhall/rs-tiled/blob/06e4ecf4257e4dd0813087e1fcd642d7f0ce7713/src/lib.rs#L167-L198

[MaulingMonkey]

It also might be worth adding a:

#[doc(hidden)] __NonExhaustive,

case to the enum. There's a whole slew of possible future extensions planned for the custom properties section:

• Enumerations (Request to Add Support for Enumeration Properties tiled#1211)
• Object references (Connecting Objects tiled#707)
• Array properties (Being able to have an array custom property tiled#1493)
• Dictionary properties (Add support for nested (dictionary-type) properties tiled#489)

...and without a __NonExhaustive case, adding new enumeration values for any of these would be a breaking change, as existing match statements become nonexhaustive.