[TypeScript 3.7] declare modifier on class fields

[thorn0]

see microsoft/TypeScript#33509

Prettier 1.18.2
Playground link

--parser typescript

Input:

class C extends B {
  declare p: 25
}

Output:

class C extends B {
  p: 25;
}

Expected behavior:

same as the input

[Cryrivers]

As discussed in #6657, we are waiting for typescript-eslint/typescript-eslint#1086.