Fixes [CMP-8028](https://youtrack.jetbrains.com/issue/CMP-8028) Text
color is sometimes randomly black on iOS when using Compose
Multiplatform (BasicText)

The main change here is to make sure that keys in `skTextStylesCache`
are immutable.
However, during the paragraph building, we need this class to be mutable
to avoid unnecessary allocations. While it's possible to fix the issue
with just a few lines (#2630), it seems really dangerous long term, so
this PR replaces `ComputedStyle` to `sealed intefrace` with both
variants to ensure immutability at compile time instead.

In addition to that, it contains a small cleanup of code around:
- Add `@InternalTextApi` to functions with `TODO: Remove from public`,
so we can actually remove them later
- Optimize cleaning up styles cache: remove in-place during iteration to
avoid allocation of extra list and additional look-ups

## Testing
- Existing tests on CI
- Reproducer from
[CMP-8028](https://youtrack.jetbrains.com/issue/CMP-8028)
```kt
val textMeasurer = rememberTextMeasurer(cacheSize = 100)
Canvas(Modifier.background(Color.DarkGray)) {
    for (i in 0 until 100) {
        drawText(
            textLayoutResult = textMeasurer.measure(text = "$i"),
            color = Color.White,
            topLeft = Offset(x = i % 10 * 50f, y = i / 10 * 50f)
        )
    }
}
```

Additional auto tests are problematic because the issue reproduces only
on K/N (both macOS and iOS) where we didn't have infrastructure for
fonts-related testing yet.

## Release Notes
### Fixes - iOS
- Fix possible text styles cache corruption (text color is sometimes
randomly black)