Add minimum supported Bash version check

[Chemaclass]

📚 Description

Add support for detecting the minimum Bash version and exiting gracefully if unsupported.

Idea from @akinomyoga, thanks!

🔖 Changes

• Documented the minimum Bash requirement in the README, stating that bashunit requires Bash 3.2 or newer
• Added a “Requirements” section in the installation guide with the same version information
• Implemented a runtime version check in the bashunit entrypoint to exit if the Bash version is below 3.2
• Logged this new feature in the changelog under Unreleased
• Created a new unit test ensuring bashunit exits with an error for unsupported versions

✅ To-do list

• I updated the CHANGELOG.md to reflect the new feature or fix
• I updated the documentation to reflect the changes

🏗️ Follow up idea

Add support for Bash 3.0 and newer

[akinomyoga]

This doesn't work. It finds bash in the PATH, which isn't necessarily the current Bash version. In particular, when I would like to test a Bash script with different Bash versions, I would do

$ bash-3.0 ./bashunit ...
$ bash-3.1 ./bashunit ...
$ ...
$ bash-5.3 ./bashunit ...

However, this _check_bash_version always sees bash in PATH, which doesn't work.

Instead, the shell variable BASH_VERSION should be used. If you can assume bash >= 2.0, you may also use BASH_VERSINFO array. Actually, you can simply check BASH_VERSINFO because if it doesn't exist, it's anyway out of support.

[Chemaclass]

I normally use zsh, so the change I think would be the best compromise (to support different terminal), is:

1. check first for the native Bash version with $BASH_VERSINFO in case such variable exists
2. Otherwise, as fallback then use the bash -v