Style file inputs with ::file-selector-button/::-webkit-file-upload-button

[easrng]

Is your feature request related to a problem? Please describe.
I don't like how the default file inputs and buttons look, marx fixes input buttons and button tags, but not the "Choose File" button.

Describe the solution you'd like
I would like to add input[type=file]::-webkit-file-upload-button, input[type=file]::file-selector-button to the selectors for the button styles.

Describe alternatives you've considered
I am currently using JS to make buttons that .click() the file inputs, but then I have to do more work

Additional context
MDN has docs on this. I would send a PR myself, but it's not clear which files are the source files. Maybe add that to the contributing section?

[jalcine]

Looks like these changes would happen at https://github.com/mblode/marx/blob/main/src/_form.css