ENH: New accessor for Index len

[Zeroto521]

• closes #xxxx
• whatsnew entry

Similar to Series.len accessor

    >>> import dtoolkit
    >>> import pandas as pd
    >>> index = pd.Index([0, 1.5, "str", ("tuple",), ["list"], {}, object])
    >>> index
    Index([0, 1.5, 'str', ('tuple',), ['list'], {}, <class 'object'>], dtype='object')
    >>> index.len()
    Float64Index([1.0, 1.0, 3.0, 1.0, 1.0, 0.0, nan], dtype='float64')

[codecov]

Codecov Report

Merging #809 (825be4c) into main (7f84090) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #809   +/-   ##
=======================================
  Coverage   99.69%   99.69%           
=======================================
  Files         141      142    +1     
  Lines        1976     1984    +8     
  Branches      428      429    +1     
=======================================
+ Hits         1970     1978    +8     
  Misses          1        1           
  Partials        5        5           

Impacted Files	Coverage Δ
dtoolkit/accessor/index/__init__.py	100.00% <100.00%> (ø)
dtoolkit/accessor/index/len.py	100.00% <100.00%> (ø)
dtoolkit/accessor/series/len.py	100.00% <100.00%> (ø)

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