add easymotion-lineforward and easymotion-linebackward

[hyhugh]

What this PR does / why we need it:

add two new easymotion commands

easymotion-lineforward and easymotion-linebackward

The behaviour of these two commands are slightly different from the original easymotion because the current easymotion implementation doesn't allow me to limit the search within a line.

Special notes for your reviewer: Sorry for the messy commit logs, i dk why the last PR's(which is already merged) commit logs are still there.

[xconverge]

Is this exactly the same as regular easymotion? Do you have some configs in your vimrc for easymotion?

I am looking here and it looks like a configurable option but only uses single leader presses not double

https://github.com/easymotion/vim-easymotion#hjkl-motions

[hyhugh]

There is no default mapping for lineforward and linebackward command in vim-easymotion. So i followed both the current vscodevim convention(which uses double leaders for word and line moving) and the configuration in the readme of vim-easymotion(which uses l and h as lineforward and backward).
btw, Actually all the default mappings for vim-easymotion use single leader.

[xconverge]

Can you add a test for each to the plugins/easymotion test please? Thanks!

[xconverge]

Thank you very much!

[hyhugh]

Thank you for merging~