Replace std::time::Instant with web_time::Instant (#14668)

# Description
The `std::time::Instant` type panics in the WASM context. To prevent
this, I replaced all uses of `std::time::Instant` in WASM-relevant
crates with `web_time::Instant`. This ensures commands using `Instant`
work in WASM without issues. For non-WASM targets, `web-time` simply
reexports `std::time`, so this change doesn’t affect regular builds
([docs](https://docs.rs/web-time/latest/web_time/)).

To ensure future code doesn't reintroduce `std::time::Instant` in WASM
contexts, I added a `clippy wasm` command to the toolkit. This runs
`cargo clippy` with a `clippy.toml` configured to disallow
`std::time::Instant`. Since `web-time` aliases `std::time` by default,
the `clippy.toml` is stored in `clippy/wasm` and is only loaded when
targeting WASM. I also added a new CI job that tests this too.

# User-Facing Changes

None.

crates/nu-system/src/windows.rs

@@ -18,7 +18,8 @@ use std::ptr;
 use std::ptr::null_mut;
 use std::sync::LazyLock;
 use std::thread;
-use std::time::{Duration, Instant};
+use std::time::Duration;
+use web_time::Instant;
 
 use windows::core::{PCWSTR, PWSTR};