Implement preferred_import_style

[lieryan]

Description

Added a new setting (imports.preferred_import_style) that controls how rope inserts new import statements.

It can be set to:

• normal-import (default) which will insert imports as import <package>
• from-module which will insert imports as from <package> import <module>
• from-global which will insert imports as from <package>.<module> import <object>

Credit: the solution here is based on nicoolas25's work on #696, but implements the config settings in a slightly different way.

Checklist (delete if not relevant):

• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have updated CHANGELOG.md
• [x] I have made corresponding changes to user documentation for new features

[codecov[bot]]

Codecov Report

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

Project coverage is 94.98%. Comparing base (54433b8) to head (0789b81).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #788 +/- ## ========================================== + Coverage 94.97% 94.98% +0.01% ========================================== Files 135 135 Lines 26405 26480 +75 ========================================== + Hits 25077 25152 +75 Misses 1328 1328 ```

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[lheckemann]

Just tested this, it works nicely and results in much smaller diffs :) thanks!

[nip3o]

@lieryan it would be pretty awesome to be able to use this. Is there something that prevents it from being merged? Anything we could contribute with?