Add multiple examples to the site

[jaredjj3]

This PR adds multiple examples to vexml.dev.

• Use different file input on small viewports.
• Make React code more idiomatic by removing unnecessary useCallback.
• Add <Select> component.
• Reads *.musicxml files from src/examples dir and wires it into the <Select> component.

[jaredjj3]

@rvilarl, just a heads up. You can add *.musicxml files to site/src/examples/**. If you have some good examples that you think ought to be there, feel free to add.