Bug: Save original index and remap after function completes

[Jeffrharr]

• closes PERF: Surprisingly slow nlargest with duplicates in the index #55767
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

Note: I'm new to this project, so this is my first PR.

Saves the index for SeriesNLargest at algorithm start and resets it before returning. This fixes performance issues when the index has many duplicate values.

Results:

• The original statistics can be viewed in the original ticket, but slow_df was several ms

In [4]: import pandas as pd
   ...: import numpy as np
   ...: 
   ...: N = 1500
   ...: N_HALF = 750
   ...: 
   ...: slow_df = pd.DataFrame({'a':  np.random.rand(N)}, index=np.concatenate([[1] * N_HALF, np.arange(N_HALF)]))
   ...: print("slow_df")
   ...: %timeit slow_df['a'].nlargest()
   ...: 
   ...: fast_df = pd.DataFrame({'a': np.random.rand(N)})
   ...: print("fast_df")
   ...: %timeit fast_df['a'].nlargest()

slow_df
427 μs ± 11.4 μs per loop (mean ± std. dev. of 7 runs, 1,000 loops each)
fast_df
420 μs ± 5.4 μs per loop (mean ± std. dev. of 7 runs, 1,000 loops each)

Tests

The existing tests should cover this unless we want to add specific tests via the asv_bench.

Addendum

I also modified the call to sort to use sort(kind="stable") to get consistent ordering which is what is currently happening in the equivalent Frame method (it was using kind=mergesort which is equivalent to kind=stable, but kept for portability). I can remove this -- it may be better in another PR.
https://numpy.org/doc/stable/reference/generated/numpy.sort.html#numpy.sort