feat(text): add operator

[pinglin]

Because

• operators are designed to be unstructured data based
• the Text operator is particularly useful for basic text operations

This commit

• remove obsolete operators
• add the text operator
• unify the naming of the text operator tasks TASK_SPLIT_TOKEN and TASK_CONVERT_TO_TEXT

[codecov]

Codecov Report

Attention: 21 lines in your changes are missing coverage. Please review.

Comparison is base (92ea699) 64.13% compared to head (4edf376) 67.93%.

Files	Patch %	Lines
pkg/text/convert.go	58.62%	8 Missing and 4 partials ⚠️
pkg/text/main.go	47.05%	6 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #39      +/-   ##
==========================================
+ Coverage   64.13%   67.93%   +3.79%     
==========================================
  Files           6        6              
  Lines         672      658      -14     
==========================================
+ Hits          431      447      +16     
+ Misses        203      168      -35     
- Partials       38       43       +5     

Flag	Coverage Δ
unittests	67.93% <56.25%> (+3.79%)	⬆️

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