Apply Flutter best practices [Non-Breaking]

[hiroshihorie]

Add flutter_lints package, and clear the lint warnings.
This PR is intended to be non-breaking, but clearing some lint errors need breaking changes, I will open another Draft PR for that.

[hiroshihorie]

@davidzhao The tests wont pass because I didn't solve the "Prefer using lowerCamelCase for constant names" lint which requires potentially breaking changes.

[davidzhao]

Ah I see. could we turn off this rule? Some of those we'd probably want to preserve, like RTCIceTransportPolicyAll

Do you have ideas around handling the print violations in example? should we ignore that folder entirely?

[hiroshihorie]

@davidzhao IMO that should be an enum maybe something like:

enum RTCIceTransportPolicy {
  all,
  relay,
}

extension RTCIceTransportPolicyExt on RTCIceTransportPolicy {
  String toStringValue() => {
    RTCIceTransportPolicy.all: 'all',
    RTCIceTransportPolicy.relay: 'relay',
  }[this]!;
}

But that's potentially breaking, I don't see references to RTCIceTransportPolicyAll and RTCIceTransportPolicyRelay as of now tho.

But it's your call 😁

[davidzhao]

that's fair. go ahead and update to your suggestions. Since we haven't officially released it and none of these enums are in the critical path, it's fine to not maintain compatibility.

[davidzhao]

looks good!

[hiroshihorie]

I find the 80 chars line length too short for flutter format, can we set it to 120?
dart-lang/dart_style#833

flutter format --set-exit-if-changed -l 120 .

[davidzhao]

I'm ok with relaxing the constraints a bit. But I think 120 is a bit too long. How about 100?

[hiroshihorie]

@davidzhao Made minor improvements to the example