Make all setter functions consistently return bytearray

[gijzelaerr]

Summary

Fixes #553

Five setter functions (set_fstring, set_string, set_dword, set_dint, set_udint) returned None while all other setters (set_bool, set_byte, set_word, set_int, set_uint, set_real, set_usint, set_sint, set_lreal, set_char, set_date, set_time) returned the modified bytearray. This inconsistency prevented idiomatic chaining like:

data = (
    set_uint(bytearray(2), 0, current_dateTime.year)
    + set_usint(bytearray(1), 0, current_dateTime.month)
    + set_udint(bytearray(4), 0, current_dateTime.microsecond * 1000)  # <-- returned None!
)

Root cause analysis

The setters were written at different times by different contributors. The earlier setters followed the pattern of returning the modified buffer, but the later additions (set_dword, set_dint, set_udint) and the string setters (set_fstring, set_string) simply forgot the return statement. Since Python functions implicitly return None, these silently broke concatenation-based patterns.

Fix

Added return bytearray_ to all five functions and updated their type annotations from -> None to -> bytearray. This is a backwards-compatible change — existing code that ignores the return value continues to work identically, since the functions still modify the bytearray in-place.

Test plan

• All 326 existing tests pass
• Verify that set_udint(bytearray(4), 0, 123) + set_uint(bytearray(2), 0, 456) now works

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