Support `exactOptionalPropertyTypes` in tsconfig `compilerOptions`

[mrclrchtr]

Description

When enabling exactOptionalPropertyTypes in tsconfig compilerOptions, I get warnings like:

TS2375: Type '{ username: string; displayName: string | undefined; }' is not assignable to type 'UserDto' with 'exactOptionalPropertyTypes: true'. Consider adding 'undefined' to the types of the target's properties.
  Types of property 'displayName' are incompatible.
    Type 'string | undefined' is not assignable to type 'string'.
      Type 'undefined' is not assignable to type 'string'. [plugin angular-compiler]

On the following code:

const userDto: UserDto = {
   username: user.username,
   displayName: user.displayName,
};

The generated type looks like:

export interface UserDto {
  displayName?: string;
  username: string;
}

A working type would look like:

export interface UserDto {
  displayName?: string | undefined;
  username: string;
}

Maybe relevant, but old issue: #590

[mrlubos]

Love this one! And we're in a much better spot to support it now. Will add