Return True from ZstdDecompressionReader.seekable

Nobody has claimed this yet.

There are several issues on this tracker discussing python-zstandard's approach to `.seek`, summarised by this comment: https://github.com/indygreg/python-zstandard/issues/216#issuecomment-2005651108

But it sounds like python-zstandard is still happy with forward seeks. If so, we should `return True` from `seekable`, so that e.g. we can forward seek through `io.BufferedReader`: https://github.com/python/cpython/blob/dd0a1f9da283bd784e2c88efec0a45cef978516a/Modules/_io/bufferedio.c#L1227

Here's a small repro:
```
import zstandard
fn = "whatever.zst"
unbuffered = zstandard.open(fn)
unbuffered.seek(10)
unbuffered.read(10) # works fine

buffered = io.BufferedReader(zstandard.open(fn))
buffered.seek(10) # fails
# UnsupportedOperation: File or stream is not seekable.
```

It's useful to wrap with BufferedReader to get functionality like `readline` (I noticed you recommended doing this here as well :-) https://github.com/indygreg/python-zstandard/issues/13#issuecomment-466797822 )

Start at ZstdDecompressionReader.seekable and reproduce the difference between direct seeking and seeking through io.BufferedReader with the example in the issue. Check the related issue discussion for the intended forward-seek behavior; done means BufferedReader.seek(10) succeeds and the subsequent read works.

Written by the indexing model from the issue text.