BREAKING CHANGE: rework on codegen and add migration feature

[si3nloong]

Changelogs:

• add migration
• refactor data type mapping

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 5.35616% with 1714 lines in your changes missing coverage. Please review.

Project coverage is 12.08%. Comparing base (7c64fec) to head (1184d05). Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
sequel/types/pgtype/array.go	0.00%	152 Missing :warning:
codegen/code_generator.go	0.00%	145 Missing :warning:
codegen/dialect/mysql/migration.go	0.00%	123 Missing :warning:
codegen/dialect/postgres/migration.go	0.00%	118 Missing :warning:
codegen/types.go	0.00%	92 Missing :warning:
sequel/types/pgtype/float_array.go	0.00%	89 Missing :warning:
sequel/types/pgtype/byte_array.go	0.00%	87 Missing :warning:
codegen/dialect/mysql/column_type.go	0.00%	82 Missing :warning:
codegen/dialect/postgres/column_type.go	0.00%	78 Missing :warning:
codegen/codegen.go	0.00%	71 Missing :warning:
... and 29 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #130      +/-   ##
==========================================
- Coverage   15.73%   12.08%   -3.65%     
==========================================
  Files          54       62       +8     
  Lines        2708     4204    +1496     
==========================================
+ Hits          426      508      +82     
- Misses       2249     3645    +1396     
- Partials       33       51      +18     

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