fix: add sliced string as a separate column

[premsrii]

StringSlicerTransformer currently slices a string in place. After this PR, it would return the original column, and the sliced string as a separate column.

For example, for the dataframe,

       foo  bar
    0  abc  jkl
    1  def  mno
    2  ghi  pqr

the transformer previously returned:

      foo bar
    0  ac  jk
    1  df  mn
    2  gi  pq

but now, it returns:

       foo  bar foo_slice bar_slice
    0  abc  jkl        ac        jk
    1  def  mno        df        mn
    2  ghi  pqr        gi        pq

[codecov]

Codecov Report

Merging #71 (8a6dce9) into develop (3983446) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop      #71      +/-   ##
===========================================
+ Coverage    99.36%   99.41%   +0.05%     
===========================================
  Files            9        9              
  Lines          469      511      +42     
===========================================
+ Hits           466      508      +42     
  Misses           3        3              

Impacted Files	Coverage Δ
src/sk_transformers/__init__.py	100.00% <ø> (ø)
src/sk_transformers/deep_transformer.py	100.00% <ø> (ø)
src/sk_transformers/generic_transformer.py	99.35% <ø> (ø)
src/sk_transformers/number_transformer.py	100.00% <ø> (ø)
src/sk_transformers/datetime_transformer.py	100.00% <100.00%> (ø)
src/sk_transformers/string_transformer.py	98.37% <100.00%> (ø)
src/sk_transformers/utils.py	100.00% <100.00%> (ø)

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