FEATURE: add `npm run` rule to prettify the code

[amtoine]

Related to https://github.com/iScsc/iscsc.fr/pull/96.

this PR adds

• a pretty rule in a Makefile
• a note in a new PR template about the "pretty" error and what to do

Note if you have things to change, please edit the PR branch directly, that'll be easier for me, that's just to launch the fix and help contributors :+1:

[amtoine]

i prefer not being assigned to this :wink:

[ctmbl]

had to fix the rule due to my experience with #86

[amtoine]

@ctmbl maybe the CI should be changed then, i think the command should be the same in both files :yum:

[ctmbl]

@amtoine it has! in #86 because the real change should have been in #95 or previously but wasn't notified then

[ctmbl]

also this is a local problem because this folder I excludes only exists in local, but I changed the CI anyway to homogenize things