universal coursier native launcher download script

[philwalk]

This is a partial implementation in response to #2647

It provides a quick way for someone running in a bash shell to download the native launcher.

It's mostly just a combination into a single script of instructions at https://get-coursier.io/docs/cli-installation, although it also explicitly supports Windows shell environment bash sessions.

There are 2 new sections on the doc page, one for the 'universal' installer, the other for Windows shell environments.
It's universal in the sense that it supports all the major operation systems, although Windows users not running in a shell environment aren't supported.

The installer script is initially in .github/scripts/install-coursier.sh, please suggest a different location if appropriate.
I'm not sure where to add the code to copy the script to the raw location to make it available at the url.

Manually verified that the script works in each of the following environments from a bash session:

Linux quip 5.15.0-78-generic #85-Ubuntu SMP Fri Jul 7 15:25:09 UTC 2023 x86_64 x86_64 x86_64 GNU/Linux
MINGW64_NT-10.0-22621 b4 3.4.9.x86_64 2023-09-15 12:15 UTC x86_64 Msys
CYGWIN_NT-10.0-22621 b4 3.4.9-1.x86_64 2023-09-06 11:19 UTC x86_64 Cygwin
Linux b4 5.15.133.1-microsoft-standard-WSL2 #1 SMP Thu Oct 5 21:02:42 UTC 2023 x86_64 x86_64 x86_64 GNU/Linux
Darwin mac.local 23.0.0 Darwin Kernel Version 23.0.0: Fri Sep 15 14:41:34 PDT 2023; root:xnu-10002.1.13~1/RELEASE_ARM64_T8103 arm64

[alexarchambault]

Thanks a lot @philwalk, that's really great work!

Give some time to ponder where the script you added should go. I put a coursier launcher script in this repository some time ago. I should mention it in the documentation, and maybe add your script alongside it, or merge it with the current one there

[philwalk]

Let me know if I can add or adjust anything