Add IANA charset output and optional lookup

[wlynxg]

Summary

• add while keeping legacy
• factor shared logic through and add encoding mapping helpers
• add optional subpackage so callers can decode via x/text
• adjust docs/tests and keep go.mod on Go 1.21 + x/text 0.22

Testing

? github.com/wlynxg/chardet/cda [no test files]
? github.com/wlynxg/chardet/consts [no test files]
? github.com/wlynxg/chardet/probe [no test files]
ok github.com/wlynxg/chardet (cached)
ok github.com/wlynxg/chardet/lookup (cached)
ok github.com/wlynxg/chardet/test (cached)