polars `xlsx_cells`

[samukweku]

PR Description

Please describe the changes proposed in the pull request:

• xlsx_cells now supports polars.

jn.xlsx_cells('../Downloads/worked-examples.xlsx',engine='polars',sheetnames='highlights',fill=True).get_column('fill')
Out[101]: 
shape: (8,)
Series: 'fill' [struct[3]]
[
        {null,{"00000000","rgb",0.0},{"00000000","rgb",0.0}}
        {null,{"00000000","rgb",0.0},{"00000000","rgb",0.0}}
        {null,{"00000000","rgb",0.0},{"00000000","rgb",0.0}}
        {null,{"00000000","rgb",0.0},{"00000000","rgb",0.0}}
        {"solid",{"FFFFFF00","rgb",0.0},{"FFFFFF00","rgb",0.0}}
        {"solid",{"FFFFFF00","rgb",0.0},{"FFFFFF00","rgb",0.0}}
        {null,{"00000000","rgb",0.0},{"00000000","rgb",0.0}}
        {null,{"00000000","rgb",0.0},{"00000000","rgb",0.0}}
]

This PR relates to #1352.

PR Checklist

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1. [x] PR in from a fork off your branch. Do not PR from <your_username>:dev, but rather from <your_username>:<feature-branch_name>.

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3. [x] Add a line to CHANGELOG.md under the latest version header (i.e. the one that is "on deck") describing the contribution.
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Relevant Reviewers

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• @ericmjl

[ericmjl]

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