fix(lsp): regression of semantic tokens of module-prefixed commands (#15603)

# Description

Fixes a regression caused by #15567, where I made the space detection in
command names switched from `get_span_content` to `get_decl().name()`,
which is slightly faster but it won't work in some cases:

e.g.
```nushell
use std/assert
assert equal
```

Reverted in this PR.

# User-Facing Changes

None

# Tests + Formatting

Refined

# After Submitting

tests/fixtures/lsp/semantic_tokens/internals.nu

@@ -13,3 +13,6 @@ export def "foo bar" [] {
 foo bar
 
 overlay use foo
+use std/assert
+
+assert equal