Relax create file lowercase validation check

[microbit-matt-hillsdon]

See https://github.com/microbit-foundation/python-editor-v3/issues/1026

[github-actions[bot]]

Preview build will be at https://review-python-editor-v3.microbit.org/relax-create-file/

[microbit-matt-hillsdon]

Code change looks ok. Needs careful review of:

• [x] Create file dialog
• [x] Project name dialog
• [x] File confirmation dialog (though should be no impact due to lack of validation)

These all share the common input dialog code that now allows for validation messages that aren't form errors.