Allow "repeat-expr" and "repeat-until" keys without "repeat" key
Requiring `repeat` key when `repeat-expr` or `repeat-until` key is specified a bit annoying. These keys are enough to understand what type of repetition is required. So I propose to make `repeat` key non-mandatory when `repeat-expr` or `repeat-until` is specified.

<details><summary>All kinds of correct input</summary>

```yaml
seq:
  - id: name
    size: 1
    repeat-expr: 3
```
```yaml
seq:
  - id: name
    size: 1
    repeat: expr
    repeat-expr: 3
```
```yaml
seq:
  - id: name
    size: 1
    repeat-until: false
```
```yaml
seq:
  - id: name
    size: 1
    repeat: until
    repeat-until: false
```
```yaml
seq:
  - id: name
    size: 1
    repeat: eos
```
</details>
<details><summary>All kinds of incorrect input</summary>

```yaml
seq:
  - id: name
    size: 1
    repeat: eos
    repeat-expr: 3
```
```yaml
seq:
  - id: name
    size: 1
    repeat: until
    repeat-expr: 3
```
```yaml
seq:
  - id: name
    size: 1
    repeat: eos
    repeat-until: false
```
```yaml
seq:
  - id: name
    size: 1
    repeat: expr
    repeat-until: false
```
</details>