Add when and unless for conditional value resolution

[phanan]

Summary

• Adds when($condition, $value, $default = null) — returns $value if $condition is truthy, otherwise $default
• Adds unless($condition, $value, $default = null) — returns $value if $condition is falsy, otherwise $default
• All three arguments can be closures: $condition is evaluated, and $value/$default are resolved accordingly

use function Functional\when;
use function Functional\unless;

when(true, 'yes');              // 'yes'
when(false, 'yes', 'no');      // 'no'
when(fn () => true, 'yes');    // 'yes'
when(true, fn () => 'computed'); // 'computed'

unless(false, 'yes');           // 'yes'
unless(true, 'yes', 'no');     // 'no'

Test plan

• Unit tests for when — truthy/falsy conditions, closures for condition/value/default (7 tests)
• Unit tests for unless — truthy/falsy conditions, closures for condition/value/default (7 tests)