shortcuts: start launcher and rename

[lazka]

Instead of calling msys2_shell.cmd call the respective launchers.
This unifies the way MSYS2 is started for most users and allows them
to set launch variables in the launcher .ini files.

While at it also rename the shortcuts to reference the environment names
instead of describing the environment. Not everyone might care or know
what these mean, so just unify them with the MSYSTEM env var which is
shown when starting bash right after.

Fixes #24

[goyalyashpal]

Instead of calling msys2_shell.cmd

One more benefit of this is that:

• calling the .cmd file runs it with the shell defined at the system environment variable CompSpec
• so, in case the user had redefined that variable to something else
  (say to choose a different shell: bash as a system wide default - rather than configuring individual terminals)
• then these scripts would open with that shell, hence will likely fail
• like they did in above mentioned issue MSYS2_shell.cmd file not launching MSYS2-packages#3102

So, setting a default shell in effect was prohibiting effective use of it. Wow!

But using .exe elevates these launcher from this unwanted dependency on CompSpec env variable.