feat(completion): stdlib virtual path completion & exportable completion (#15270)

# Description

More completions for `use` command.

~Also optimizes the span fix of #15238 to allow changing the text after
the cursor.~

# User-Facing Changes

<img width="299" alt="image"
src="https://github.com/user-attachments/assets/a5c45f46-40e4-4c50-9408-7b147ed11dc4"
/>

<img width="383" alt="image"
src="https://github.com/user-attachments/assets/fbeec173-511e-4c72-8995-bc1caa3ef0d3"
/>


# Tests + Formatting

+3

# After Submitting

tests/fixtures/lsp/completion/use.nu

@@ -0,0 +1,8 @@
+export module "🤔🐘" {
+  export const foo = "🤔🐘";
+}
+
+export use std-rf
+export use std-rfc/clip [ copy, paste ]
+use std null_d
+use 🤔🐘 [ foo, ]