fix: add checking for missing extensions

[jaeheonji]

Fixes #1431

Under the circumstance of checking the existence of a layout file, the default assets are always loaded due to missing extensions.

[a-kenji]

LGTM,
thank you for the quick fix!

[jaeheonji]

@a-kenji Thank you for reviewing :)

[raphCode]

I think this commit slightly changed the behavior:
Previously, I had a layout file standard without file extension in the layout directory that I could load with zellij --layout standard.
This command now failed with IoError: The layout was not found, File: standard until I renamed the file to standard.yaml.

Not sure if I did something unusual but I feel we should mention the behavior change and fix (rename to .yaml) in the changelog?

[jaeheonji]

I think the commit that changed the behavior is related to #1413 .

This commit is close to solving the problem that the existing function did not work properly because there was a missing part due to behavioral changes.