[clang-format] Handle C# 9 `init` accessor specifier.

Before, the code:
```
int Value { get; } = 0;
int Value { init; } = 0;
```
was formatted incoherently:
```
int Value { get; } = 0;
int Value { init; }
= 0;
```
because `init` was not recognised as an accessor specifier.

Reviewed By: MyDeveloperDay, HazardyKnusperkeks

Differential Revision: https://reviews.llvm.org/D121132