refactor: merge some encoding and encoding names related code

[shenlebantongying]

Now, Iconv reuses Text's encoding names.

[sonarqubecloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[xiaoyifang]

maybe. use https://doc.qt.io/qt-6.2/qstringconverter.html#Encoding-enum directly .
and avoid all the conversion.

[shenlebantongying]

There are two issues 1) we need a few more encoding names 2) In a few places, the enum's underlying number is written into the index files.