Support reading/extracting existing chapters from MKV files

[xMohnad]

Is your enhancement request related to a problem? Please describe.
pymkv2 can add chapters to MKV files, but it cannot read chapters that already exist in a file.

Describe the solution you'd like

Add support for parsing and exposing chapter data from MKV files.

Implementation steps

• Detect chapters in MKV files
• Parse chapter data
• Convert it to the Chapter object
• Populate "MKVFile.chapters_obj" on load
• Add tests for files with and without chapters

Additional context

Example:

mkvextract chapters "foo.mkv"

This shows chapters in XML, but:

from pymkv import MKVFile

mkv = MKVFile("foo.mkv")
print(mkv.chapters_obj)  # None

Expected: "chapters_obj" should contain the parsed chapters.