Add support for clap `unstable-dynamic` feature

[baka-gourd]

Problem description

Although this feature is marked as “unstable,” it is actually usable and has already been implemented in cargo and rustup.

Why

In short: This can significantly speed up Source. Take pwsh as an example: currently, pixi completion requires generating and running a script that’s nearly 707 KB, but with dynamic completion enabled, it only requires a script of less than 1 KB. This benefits all terminals.

[Cloud0310]

I agreed with this @baka-gourd 's idea with enabling dynamic completion, as this both provides better completion experience and better sourcing experience. And I've done the adoption for rustup in rust-lang/rustup#5033, and cargo team also supported this in pr.

This is the issue me and @baka-gourd met when trying to optimizing the performance for clap pwsh completion performance. Then we found out that on Windows, the pwsh completion script of pixi is oversized, which is the performance bottleneck.