CLI does not work with dart 2.10

In the cli folder:

```
~/tmp/figma-to-flutter/figma_to_flutter/cli (master) $ dart pub get
Resolving dependencies...
The current Dart SDK version is 2.10.3.

Because every version of figma_to_flutter from path depends on source_gen >=0.0.0+1 <=0.8.3 which requires SDK version
>=1.0.0 <2.0.0 or >=2.0.0-dev.19.0 <2.0.0, figma_to_flutter from path is forbidden.
So, because figma_to_flutter_cli depends on figma_to_flutter from path, version solving failed.
```

In the src folder:
```
~/tmp/figma-to-flutter/figma_to_flutter/src (master) $ dart pub get
Resolving dependencies...
The current Dart SDK version is 2.10.3.

Because figma_to_flutter depends on code_builder >=0.1.3 <3.1.2 which requires SDK version >=1.9.1 <2.0.0-∞, version solving
failed.
```