Replace mecab-python3 with fugashi for Japanese tokenization (#6086)

* Replace mecab-python3 with fugashi

This replaces mecab-python3 with fugashi for Japanese tokenization. I am
the maintainer of both projects.

Both projects are MeCab wrappers, so the underlying C++ code is the
same. fugashi is the newer wrapper and doesn't use SWIG, so for basic
use of the MeCab API it's easier to use.

This code insures the use of a version of ipadic installed via pip,
which should make versioning and tracking down issues easier.

fugashi has wheels for Windows, OSX, and Linux, which will help with
issues with installing old versions of mecab-python3 on Windows.
Compared to mecab-python3, because fugashi doesn't use SWIG, it doesn't
require a C++ runtime to be installed on Windows.

In adding this change I removed some code dealing with `cursor`,
`token_start`, and `token_end` variables. These variables didn't seem to
be used for anything, it is unclear to me why they were there.

I ran the tests and they passed, though I couldn't figure out how to run
the slow tests (`--runslow` gave an error) and didn't try testing with
Tensorflow.

* Style fix

* Remove unused variable

Forgot to delete this...

* Adapt doc with install instructions

* Fix typo

Co-authored-by: sgugger <sylvain.gugger@gmail.com>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

setup.py

@@ -65,7 +65,7 @@ if stale_egg_info.exists():
 
 extras = {}
 
-extras["mecab"] = ["mecab-python3<1"]
+extras["ja"] = ["fugashi>=1.0", "ipadic>=1.0,<2.0"]
 extras["sklearn"] = ["scikit-learn"]
 
 # keras2onnx and onnxconverter-common version is specific through a commit until 1.7.0 lands on pypi
@@ -97,7 +97,7 @@ extras["quality"] = [
     "isort @ git+git://github.com/timothycrosley/isort.git@e63ae06ec7d70b06df9e528357650281a3d3ec22#egg=isort",
     "flake8",
 ]
-extras["dev"] = extras["testing"] + extras["quality"] + ["mecab-python3<1", "scikit-learn", "tensorflow", "torch"]
+extras["dev"] = extras["testing"] + extras["quality"] + extras["ja"] + ["scikit-learn", "tensorflow", "torch"]
 
 setup(
     name="transformers",