Allow more characters in field names (once more, with feeling)

[Deleted user]

Vaguely related to #48, mysql allows fields starting with numbers. it also supports the full unicode BMP except U+0000, although that's not a pressing issue for me at present.

• mysql docs: https://dev.mysql.com/doc/refman/8.0/en/identifiers.html
• reproduction: $db->escapdeIdentifier('50pence');