Custom Command: Launch .lnk files using Proton automatiically

[sonic2kk]

When trying to use .lnk files as custom commands, SteamTinkerLaunch will default to interpreting these as native games, because it does not return the PE32 string from "Files" and it is not a .bat script. Since this is not a native program it will try to run this with whatever program is set up on the system to handle files of this type. Commonly this is Wine but if a user doesn't have Wine installed it could end up opening a text editor, a hex editor, or anything really.

As a workaround it is possible to force Proton since 77e0772 (no PR for this one apparently). But this is not ideal.

To fix this, we check if the custom command ends with .lnk and if so we catch this as a custom command to run with Proton. I tested this with Touhou 16 and it fixed the problem. Without this PR it tries to use the system Wine to run the file.

We could've used file here to check for MS Windows shortcut but I opted to check the extension for simplicity and just in case this ever returns a different string (PE32 is universal and virtually guaranteed to never change). This isn't perfect because if a filename has .lnk anywhere we'll try to use Proton even if it's not a Windows shortcut. But we do the exact same check for .bat files and it could have the exact same pitfalls, but that has not yet been reported.

If either of these ever become issues though we can use a different pattern to match the end of the string with the extension.

NOTE: Custom commands do not use the Steam Linux Runtime, but maybe this will change in the near future, as I've run into a few cases recently where I want to use custom commands to replace a game launch, and the lack of SLR has caused issues.

TOD:

• Further testing to ensure regular custom commands (native and Proton) still work
• Version bump

[sonic2kk]

Tested this pretty thoroughly by using it for all games since this PR was opened, including games using custom commands (EXEs and shell scripts), and it works fine. This can be merged pending a version bump.