Updated linux bash completion

[karnigen]

It appears that the current vcpkg Bash autocompletion is quite outdated and doesn't function correctly. I'm submitting this pull request with improvements for Linux. I'm not certain if other operating systems need similar modifications.

[BillyONeal]

This needs to be changed in https://github.com/microsoft/vcpkg/blob/master/scripts/vcpkg_completion.bash rather than added as a new file.

[karnigen]

I'm using bash-completion version 2.11 (2020), and the current version is 2.16 (2024). Older versions of bash-completion might work with the original vcpkg_completion.bash file, so I've created a new one. I don't have the ability to verify different versions of bash-completion.

[karnigen]

@microsoft-github-policy-service agree

[karnigen]

In last commit:

• Refined bash completion with improved flag completion;
• Reverted to the original filename scripts/vcpkg_completion.bash.
• Does not require the presence of external bash-completion routines to function.
• Tested on Bash 5.1..