feat: implement write

[Sl1mb0]

Closes #337

• feat: implement write
• test: add unit tests for write functionality There are notably two test cases that we do not test: - Writing an empty buffer to a directory - Writing to a directory The reason we do not test these is because these errors are caught by open_at; which is required to be called before we can perform writes (a descriptor is required); hence it's impossible to get a write descriptor for a directory.
• refactor: return ErrorCode::Unsupported for write_via_stream to indicate that it is not currently implemented
• test: add integration test for write behavior Notably, this asserts that if a write is attempted by the python guest we will see ErrorCode::Unsupported. This is because under the hood Python uses write_via_stream; and not write.

[Sl1mb0]

Just want to highlight this one. I'll implement write_via_stream in a follow-up.