[Formats] Add support for Steam appmanifest *.acf files

[Swiss-Mac-User]

The Steam's appmanifest files are basically plain text files, but obfuscated using the file extension acf. They can be opened with TextEdit (or any other text editor) in macOS.

However, the Syntax Highlight is not able to parse it - showing only a fallback document icon and meta information.

Output from Inquiry format in Syntax Highlighter.app:

Here's an example from a Steam appmanifest acf-file contents from within TextEdit: