Fix TS problem and other

[Mara-Li]

Hello! I fixe several problems in the TS code :

• Use checkCallback() to prevent the use of a checking during command : these commands won't appear if there is no active file (md or canvas)
• Fix a eslint rules problem
• integrate cross-env for building on Windows
• Use const instead of let
• Some inline variable/simplifies expressions.

[trganda]

Tks!!! Your are so great, seriously I'm no good at ts or js.