Windows/MSVC: Use hybrid CRT by default

Now that the tier-1 Windows targets only support Windows 10 and up, I suggest that they use Microsoft's Hybrid CRT configuration by default. This way, any Rust user can easily distribute a standalone executable that runs on any Windows 10+ machine without having to install a specific version of the VC++ runtime, because that runtime is statically linked. But the UCRT is still dynamically linked, so the impact on executable size is minimal.

This can already be done in .cargo/config.toml as follows:

[target.'cfg(target_env = "msvc")']
rustflags = ["-C", "target-feature=+crt-static", "-C", "link-arg=/nodefaultlib:libucrt.lib", "-C", "link-arg=ucrt.lib"]

IMO, even though it's already possible to do this via .cargo/config.toml, it should be the default. That will eliminate one papercut that gives Windows native desktop development a reputation for being hard.

My company has shipped a desktop application using this configuration to thousands of Windows machines for over a year now, and this configuration hasn't caused any problems.

Here are executable sizes for a hello-world program compiled for x86_64-pc-windows-msvc with Rust 1.94.0:

• Fully dynamic CRT: 127 KB
• Hybrid CRT: 153 KB
• Fully static CRT: 226 KB

I think the Hybrid CRT option is the sweet spot: just-works distribution with modest impact on executable size.

Start with the target-specific MSVC settings shown for .cargo/config.toml and trace how Rust applies default linker and CRT configuration for tier-1 Windows targets. Done means the hybrid CRT behavior is the default for supported Windows 10+ MSVC targets, without requiring users to add the configuration themselves.