🚀 Add support for `hunk-label-style`

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Currently, the color of `hunk-label` respects `hunk-header-file-style`, which defaults to blue. This produces a consistent coloring when the filename is included in the hunk headers, via the special "file" keyword:

```toml
[delta]
# ...
hunk-header-style = "file line-number syntax"
hunk-label = "Foobar"
hunk-header-line-number-style = "red"
```


But if I don't include "file" in `hunk-header-style`, I would like to set the hunk label to match the color of the remainder of the hunk header (in this case, the line number). I assumed there would be a `hunk-label-style` config to do so, but that doesn't seem to be the case:

```toml
[delta]
# ...
hunk-header-style = "line-number syntax"
hunk-label = "Foobar"
hunk-label-style = "red"
hunk-header-line-number-style = "red"
```


I can work around this by setting `hunk-header-file-style`:

```toml
[delta]
# ...
hunk-header-style = "line-number syntax"
hunk-label = "Foobar"
hunk-header-line-number-style = "red"
hunk-header-file-style = "red"
```


...but that's awkward because I'm not using "file" in the hunk header at all.

Ideally, there should be a dedicated `hunk-label-style` for styling the hunk prefix in the hunk headers.

(By the way, why is this config called `hunk-label` instead of `hunk-header-label`? That would be more consistent with the other configurations. And if the name were that, then the associated style config would be `hunk-header-label-style`.)

Start by tracing how the existing hunk-label and hunk-header-file-style configuration options are defined and applied. Done means a dedicated hunk-label-style option controls the hunk prefix independently, while the issue's configuration examples produce the requested matching colors.

Written by the indexing model from the issue text.