Feature/fix remaining welcome commands

[akinsho]

This tackles some of the left over issues re. the welcome as described by @CrossR as well as adds a command to open the welcome screen and I discovered a nice way to check if a key is remapped in vim which was necessary for the : command so I added it to the neovimInstance and editor for future use

[codecov[bot]]

Codecov Report

Merging #2533 into master will increase coverage by 0.12%. The diff coverage is 34.14%.

@@            Coverage Diff             @@
##           master    #2533      +/-   ##
==========================================
+ Coverage   45.31%   45.43%   +0.12%     
==========================================
  Files         353      353              
  Lines       14426    14444      +18     
  Branches     1885     1885              
==========================================
+ Hits         6537     6563      +26     
+ Misses       7666     7657       -9     
- Partials      223      224       +1

Impacted Files	Coverage Δ
browser/src/neovim/NeovimInstance.ts	5.47% <0%> (-0.08%)	:arrow_down:
browser/src/Editor/NeovimEditor/NeovimEditor.tsx	8.82% <0%> (-0.02%)	:arrow_down:
...ser/src/Editor/NeovimEditor/WelcomeBufferLayer.tsx	68.21% <40%> (+12.81%)	:arrow_up:

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[akinsho]

@CrossR @TalAmuyal would appreciate if either of you could review as this I think (for now ) ties up the work I was doing on the welcome screen it hooks up some more commands and adds a command to the quick open that allows a user to reopen the welcome screen

[akinsho]

@CrossR yeah was wondering whether or not it would be good to have on by default at this point.

The sessions bit though should stay as experimental atm, its possibly got a weird bug where on occasion WIP is disappearing, frankly dont know how thats possible since all we do is call mksession and source ${session} the latter of which is a very visible command and isnt being called, but somehow wip is still occasionally disappearing although this seems to happen with it disabled either way so not sure if its the cause 🤷‍♂️ 😕