Allow specifying the use of a file extension for StandaloneLinux64

[kitlith]

Context

Currently, when building a game for StandaloneLinux64, this action will produce an executable without a file extension. This is fine, but is a mismatch for what the unity editor will do by default (produces an executable with the extension .x86_64), may cause some packages to build incorrectly (https://github.com/ValveSoftware/unity-xr-plugin/issues/139), and technically violates unity's (only recently?) documented build path requirements.

Considered solutions

Keeping in mind that other people may be relying on this lack of a file extension, we cannot simply change the default without possibly causing breakage.

The least configurable option would be to add a parameter bikeshedLinuxUseFileExtension: true with a default to false which causes the file extension to be .x86_64 when building for StandaloneLinux64. (or as appropriate for StandaloneLinux32/older versions of unity.) In the future, it may be viable to flip the default.

Another option would be to add a parameter bikeshedLinuxFileExtension: string which sets the file extension, allowing the user to configure it to anything.

Considered alternatives

Fixing https://github.com/ValveSoftware/unity-xr-plugin/issues/139 upstream or downstream.

[kitlith]

Per conversation with a maintainer on discord, now leaning towards the boolean approach, though I've flipped the meaning of the boolean: linux64RemoveExecutableExtension, currently defaulting to true, and flip the default to false upon the next version bump.