Modified some edge cases and migrated to null-safety

[ngxingyu]

Implemented the following regex logic:

• Ensure swallow only removes leading 'export' keyword
• Retain leading and trailing spaces within quotes (i.e. X = " <-space" returns X: " <-space" and not X: "<-space")
• No quotes: Keep entire string, strip whitespaces
• Quotes: retain spaces within quotes
• No or double quotes: Replace (interpolate) $VAR or ${VAR} without leading backslash with existing env vars.
• Only read until the first matching non escaped quote i.e. " \" ' ' ' \" all included until ->" # to allow for comments
• Remove single backslashes before $, ', " and \n

[ngxingyu]

Migrated to null-safety and moved test folder to the root folder. Updated changelog. I'm not sure if the backslash escape is what's intended or if there are any drawbacks to that approach, so do let me know if there's anything that can be improved.

[java-james]

Thanks a lot for your PR. I will take a look.

[tusharsadhwani]

@java-james any updates? Null safety has been in beta for quite a while now, it'll be really nice if all packages move to it

[nstrelow]

@java-james Please merge this at some point. THank you :D

[java-james]

Thanks have made time to have a look at this. Looks good. A couple of very minor changes in review. Let me know if you don't have time and I will make time to push the changes. Should be able to commit suggestions for all of them? Then good to go

[java-james]

@java-james Please merge this at some point. THank you :D

Agree we should get this in. Have made time to take a look. Will release after merge. Thanks

[java-james]

@ngxingyu Thank you for taking a look at the review. I will get this merged and released this evening.

[java-james]

Published to pub.dev as 4.0.0-nullsafety.0

Thanks @ngxingyu for spending the time to migrate.