builtin/pack-refs: factor out core logic into a shared library

The implementation of `git pack-refs` is monolithic within
`cmd_pack_refs()`, making it impossible to share its logic with other
commands. To enable code reuse for the upcoming `git refs optimize`
subcommand, refactor the core logic into a shared helper function.

Split the original `builtin/pack-refs.c` file into two parts:

- A new shared library file, `pack-refs.c`, which contains the
  core option parsing and packing logic in a new `pack_refs_core()`
  helper function.

- The original `builtin/pack-refs.c`, which is now a thin wrapper
  responsible only for defining the `git pack-refs` command and
  calling the shared helper.

A new `pack-refs.h` header is also introduced to define the public
interface for this shared logic.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: shejialuo <shejialuo@gmail.com>
Signed-off-by: Meet Soni <meetsoni3017@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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