GH-49237: [R] Deprecate Feather reader and writer

[thisisnic]

Rationale for this change

Arrow C++ is deprecating the Feather reader/writer (#49231) so we should update the R functions too

What changes are included in this PR?

• Updated write_feather to mention in the docs that it's deprecated.
• Refactored write_feather() to wrap new function write_ipc_impl() so we can show deprecation warnings on calls to write_feather() only (previously write_ipc_file() called write_feather() and so I didn't wanna add the deprecation warning without this refactor otherwise a call to write_ipc_file() would trigger it erroneously)
• Once we eventually remove write_feather() we can remove this extra function

• split read_ipc_file() and read_feather() into separate functions so we can document them differently. Updated the examples to show using read_ipc_file()

Some 🤖 use here, but I decided on what to do.

Are these changes tested?

Yep

Are there any user-facing changes?

Yep - deprecation warning

• GitHub Issue: [R] Deprecate Feather reader and writer #49237

[github-actions]

⚠️ GitHub issue #49237 has been automatically assigned in GitHub to PR creator.