Allow trailing commas in constructor initializer lists and generic type lists

Trailing comma is supported in many places like argument lists, list entries, etc. However there are still places where it is not supported by would be beneficial:

1. Constructor initializer:

```
PlatformInstance()
:
_environment = Platform.environment,
_executable = Platform.executable,
_executableArguments = Platform.executableArguments,
_localeName = Platform.localeName,
_localHostname = Platform.localHostname,
_numberOfProcessors = Platform.numberOfProcessors,
_operatingSystem = Platform.operatingSystem,
_operatingSystemVersion = Platform.operatingSystemVersion,
_packageConfig = Platform.packageConfig,
_pathSeparator = Platform.pathSeparator,
_resolvedExecutable = Platform.resolvedExecutable,
_script = Platform.script,
_version = Platform.version, // <========
;
```

2. Generic type lists:
```
class GenericClass<
A,
B extends SomeClass,
C extends AnotherClass, // <=======
> {
}
```

Proposal: to allow trailing comma in these cases to minimize changes when adding or moving new lines there.

Start with issue #1402 and compare its two examples with the existing trailing-comma cases described in the issue. Determine the language changes needed for constructor initializer lists and generic type lists, then verify that both forms accept a final comma without changing other syntax rules.

Written by the indexing model from the issue text.