A builtin returning the Bash version as a Num

[lens0021]

It's out of scope for this PR, but we should probably write a builtin returning the Bash version as a Num.

Originally posted by @hdwalters in #620 (comment)

Along with a builtin to return the Bash version (suggested in a comment on a different PR) we should also add an annotation specifying the minimum Bash version to support a given function or main block.

Originally posted by @hdwalters in #616 (comment)

[hdwalters]

Thanks for creating this issue. I would have got round to it eventually!

[lens0021]

I think it should expose the fix version as well as the major and the minor.

[hdwalters]

I'm running Bash 5.2.21.1; the Amber function I wrote to bypass negative array index validity tests returns 502 for me:

#[allow_absurd_cast]
fun bash_version(): Num {
    let major = trust $ echo "\$\{BASH_VERSINFO[0]}" $ as Num
    let minor = trust $ echo "\$\{BASH_VERSINFO[1]}" $ as Num
    return (major * 100) + minor
}

main {
    if bash_version() >= 402 {
    ...

If we're going to include all four numeric components, extending this to 5022101 or 5002021001 will be unworkable.

Can I suggest that we consider returning an array, and implementing version comparison, at least for [Num] arrays:

let bashver = [5, 2, 21, 1]
if bashver >= [4, 3] {
    // Bash 4.3 or above
}