registerOPFSFileName and relevant .files register in the shell

[carlopi]

Allows, in the shell, to do:

.files register opfs://a.csv
COPY (SELECT 32) TO 'opfs://a.csv';

close tab, restart, then:

.files register opfs://a.csv
FROM 'opfs://a.csv';

Roadmap:

• Make registration of existing files automatic
• Allow clean-up of file
• Allow listing of files (and list them on startup)
• Allow reset of OPFS storage
• Make error situation clear in multi-tab scenario