feature request: `OilDefaultFileIcon` hl group

[simonmandlik]

Did you check existing requests?

• I have searched the existing issues

Describe the feature

Following #145, could we please add a OilDefaultFileIcon hl group?

Willing to provide a PR

What is the significance of this feature?

nice to have

[simonmandlik]

There are actually two ways to do it:

• either fall back to the proposed OilDefaultFileIcon here if hl is nil,
• or call devicons.get_icon with { default = true }, using the default icon and hl configurable in devicons

I would prefer the first option because the second one makes conf.default_file not needed.