Improved Split fields: option to keep the field to split and append new fields

[leehaut]

Fix #3820

Summary

• Add a Keep the field to split option to the Split fields (FieldSplitter) transform.
• When enabled, the original field is kept and the new fields are appended at the end of the field list
• When disabled (default), behavior is unchanged: the original field is replaced in place.
• Move the former 0054-split-fields-* integration tests into the 0003-field-splitter-* suite and add a unit/integration test covering the keep-field path.
• Update user documentation and en_US / zh_CN messages; simplify FieldSplitterMeta with Lombok @Getter / @Setter.

Behavior

Input
id | value | value2 with value = 500,300,200,100
Default (keep_split_field = N)
id | sales1 | sales2 | sales3 | sales4 | value2
Keep enabled (keep_split_field = Y)
id | value | value2 | sales1 | sales2 | sales3 | sales4