Dart null safty and fix for freeze

[stha-ums]

Following are the things I worked on:

• added detect enum option
• added null-safety
• make all properties nullable
• keep properties name, enum, and union name same as Key in JSON (this is useful while generated for Freezed with JsonSerializer) .

I have done manual testing. So it should be working. For automatic testing. I do not have enough knowledge. Is anyone can test or guide me. That would be great.

[NatoBoram]

• added null-safety

Closes #1673, #1692, #1711, #1723, #1729, #1737, #1739, #1741, #1744, #1747, #1748, #1751, #1753, #1755, #1758, #1760, #1769, #1783, #1785, #1786, #1792, #1794, #1799, #1800, #1801, #1802, #1810, #1812, #1830

Fun fact : Since #1673, about 18% of this repo's issues are about Dart's Null Safety.

[schani]

@stha-ums Thank you for this contribution! There's a lot of reformatting of unrelated files in this PR, as well as huge changes to package-lock.json. It also breaks the Elm tests - why does it touch Elm at all?

[NatoBoram]

@schani Looks like someone on your team misused the package.json file by manually entering/ordering dependencies instead of using npm. The dependencies in package.json are generally in the same order to avoid this kind of formatting issue.

As for elm-package.json, is it an auto-generated file? This project includes prettier. If someone uses this project's settings, it is reasonable that prettier would be applied.

That's the explanation I was able to find regarding the reformatting in this PR.

[stha-ums]

@schani there was an error while building that project with elm versioning so I had to change the version to run on my machine. I am relatively new to Node, what do you suggest for the resolution?

[NatoBoram]

@stha-ums Can you edit this PR to keep their original config, even if it doesn't build on your machine? It's clear they don't have much time to give to this project, so if it works on their machine I think it's good enough.

[stha-ums]

sent another PR #1764 .