When converting a slice of a nested struct alias, the type is incorrectly inferred as readonly unknown[] instead of the expected struct type.

Environments
Go version: go version go1.23.8 darwin/arm64
guts version: v1.3.0

Steps to Reproduce:

1. Add the following Go code to testdata/alias/alias.go:

   package alias
   
   type FooStruct struct {
   	Key string
   }
   
   type AliasStruct = FooStruct
   type AliasStructNested = AliasStruct
   type AliasStructSlice = []FooStruct
   type AliasStructNestedSlice = []AliasStructNested

2. Run the test for the alias generation:

   go test -run "TestGeneration/alias"

Actual Behavior:

The test fails with the following diff, showing AliasStructNestedSlice is converted to readonly unknown[]:

--- Expected
+++ Actual
@@ -15,2 +15,23 @@
 // From alias/alias.go
+export interface AliasStruct {
+    readonly Key: string;
+}
+
+// From alias/alias.go
+export interface AliasStructNested {
+    readonly Key: string;
+}
+
+// From alias/alias.go
+export type AliasStructNestedSlice = readonly unknown[];
+
+// From alias/alias.go
+export type AliasStructSlice = readonly FooStruct[];
+
+// From alias/alias.go
 export type Foo = string;
+
+// From alias/alias.go
+export interface FooStruct {
+    readonly Key: string;
+}

Specifically, the generated TypeScript for AliasStructNestedSlice is:

// From alias/alias.go
export type AliasStructNestedSlice = readonly unknown[];

Expected Behavior:

The AliasStructNestedSlice should be converted to a slice of the aliased struct type.

Potential Cause and Workaround:

It seems the issue might be related to how types.Alias is handled. In the following code snippet

If ty.Underlying() is changed to ty.Rhs(), AliasStructNestedSlice is converted to readonly FooStruct[]. The diff for the test results with this change is:

--- Expected
+++ Actual
@@ -15,2 +15,23 @@
 // From alias/alias.go
+export interface AliasStruct {
+    readonly Key: string;
+}
+
+// From alias/alias.go
+export interface AliasStructNested {
+    readonly Key: string;
+}
+
+// From alias/alias.go
+export type AliasStructNestedSlice = readonly FooStruct[];
+
+// From alias/alias.go
+export type AliasStructSlice = readonly FooStruct[];
+
+// From alias/alias.go
 export type Foo = string;
+
+// From alias/alias.go
+export interface FooStruct {
+    readonly Key: string;
+}

While changing ty.Underlying() to ty.Rhs() resolves the immediate issue of unknown[], I'm unsure whether AliasStructNestedSlice should be converted to readonly AliasStructNested[] or readonly FooStruct[]. Further discussion on the desired behavior for nested aliases would be beneficial.