Feat: sessions added to startify page

[zhengrc19]

TLDR: Looks if neovim-session-manager is installed and if there are saved sessions. If both suffice, a new list of sessions is added on top of MRU CWD and MRU in the startify theme. This is supposed to solve #51 and #72.

Checks for installation of neovim-session-manager in lua/alpha/themes/startify.lua:162.

Creates buttons (with optional icons) according to the session filenames lua/alpha/themes/startify.lua:164-202.

Changes the numerical shortcuts according to number of sessions found in lines lua/alpha/themes/startify.lua:229,243.

Adds sessions to section in lua/alpha/themes/startify.lua:212, and adds this section.sessions to config.layout in lua/alpha/themes/startify.lua:267.

The icons are turned off by default, configured in lua/alpha/themes/startify.lua:61.

P.S. This is my first PR ever so if there are any bad practices please point them out to me. Thanks!

[itamark-targa]

@zhengrc19 Thanks! very helpful, that's the 1 feature I was really missing moving from vim to nvim.
I have a question - is there a way to have sessions saved based on named-session, and not based on a directory? my sessions are just group of files related to a single task/project, so saving based on directory is less convenient.

== EDIT === - never mind, I found out that the original "startify" plugin for Vim works also for Neovim, so that solves all of my problems :)

[zhengrc19]

@zhengrc19 Thanks! very helpful, that's the 1 feature I was really missing moving from vim to nvim. I have a question - is there a way to have sessions saved based on named-session, and not based on a directory? my sessions are just group of files related to a single task/project, so saving based on directory is less convenient.

== EDIT === - never mind, I found out that the original "startify" plugin for Vim works also for Neovim, so that solves all of my problems :)

Hey @itamark-targa thanks for reaching out! Yes in fact I was using vim-startify on Neovim for quite a while and it definitely has all these features implemented. I see if I can implement the session naming feature into alpha-nvim in the future. Do you know how vim-startify does this, does it even utilize neovim-session-manager?

[goolord]

vim-startify basically implements its own session management plugin, which is something i intentionally didn't include in alpha

[goolord]

i think i like this solution, i'll have to test it / see how it impacts statup times and such