fluent API for snapshotting only specific subsets of a snapshot

It would be nice to be able to truncate a snapshot to only certain specific lines. Something like:

```java
expectSelfie(blah).lines(1).toBe_TODO() // first line
expectSelfie(blah).lines(2).toBe_TODO() // first two lines
expectSelfie(blah).lines(-2).toBe_TODO() // last two lines
expectSelfie(blah).lines(-1).toBe_TODO() // last one line

var snapshot = expectSelfie(blah)
snapshot.lines(2).toBe_TODO() // first two lines
snapshot.lines(-2).toBe_TODO() // last two lines
```

I have also sometimes wanted to match a certain regex, like maybe

```
expectSelfie(blah).withinRegex("(?m)^.*$\n?^.*$").toBe_TODO()
```

So far, each time that I have wanted this, I have found a better way to solve this by filtering before the snapshot gets taken...

まず既存の expectSelfie API と、スナップショットの値がどのように照合されるかを調査します。行の選択と regex マッチングが引き続き必要かどうかを明確にし、それらのセマンティクスを定義して、実装箇所とテスト箇所を特定する前に Fluent API について合意します。

索引モデルが issue の本文から書いたものです。