Introduce exported PictureInsertType enumeration

[yyle88]

PR Details

Description

Related Issue

Motivation and Context

How Has This Been Tested

Types of changes

• [ ] Docs change / refactoring / dependency upgrade
• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• [ ] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.21%. Comparing base (838232f) to head (949ae6e).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1864 +/- ## ======================================= Coverage 99.21% 99.21% ======================================= Files 32 32 Lines 24031 24033 +2 ======================================= + Hits 23842 23844 +2 Misses 101 101 Partials 88 88 ``` | [Flag](https://app.codecov.io/gh/qax-os/excelize/pull/1864/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qax-os) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/qax-os/excelize/pull/1864/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qax-os) | `99.21% <100.00%> (+<0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qax-os#carryforward-flags-in-the-pull-request-comment) to find out more.

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