Support Linux, macOS, Windows

[vitusortner]

Adds support for Linux, macOS (which didn't seem to work before 🤔) and Windows by using sqflite's ffi implementation when needed.

Closes #228

[codecov]

Codecov Report

Merging #326 into develop will decrease coverage by 0.19%.
The diff coverage is 54.54%.

@@             Coverage Diff             @@
##           develop     #326      +/-   ##
===========================================
- Coverage    81.67%   81.47%   -0.20%     
===========================================
  Files           58       59       +1     
  Lines         1495     1506      +11     
===========================================
+ Hits          1221     1227       +6     
- Misses         274      279       +5     

Flag	Coverage Δ
#floor	81.47% <54.54%> (-0.20%)	⬇️
#floor_generator	81.13% <ø> (ø)

Impacted Files	Coverage Δ
..._generator/lib/writer/database_builder_writer.dart	100.00% <ø> (ø)
floor_generator/lib/writer/database_writer.dart	80.55% <ø> (ø)
floor/lib/src/sqflite_database_factory.dart	54.54% <54.54%> (ø)

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