[Feature] Support 'python -m mim' usage

[ice-tong]

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Motivation

Add mim/__main__.py to support python -m mim usage.

Modification

• mim/__main__.py

[codecov]

Codecov Report

Base: 55.58% // Head: 55.44% // Decreases project coverage by -0.14% ⚠️

Coverage data is based on head (4da01d1) compared to base (f989842).
Patch coverage: 0.00% of modified lines in pull request are covered.

❗ Current head 4da01d1 differs from pull request most recent head 897b4d7. Consider uploading reports for the commit 897b4d7 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #158      +/-   ##
==========================================
- Coverage   55.58%   55.44%   -0.15%     
==========================================
  Files          23       24       +1     
  Lines        1567     1571       +4     
  Branches      333      334       +1     
==========================================
  Hits          871      871              
- Misses        566      570       +4     
  Partials      130      130              

Flag	Coverage Δ
unittests	55.44% <0.00%> (-0.15%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mim/__main__.py	0.00% <0.00%> (ø)

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