refactor: add rule.* type assertions (#1961)

**What type of PR is this?**

> Other

**What package or component does this PR mostly affect?**

> all

rule/platform_strings.go

@@ -68,6 +68,8 @@ type PlatformStrings struct {
 	Platform map[PlatformConstraint][]string
 }
 
+var _ BzlExprValue = (*PlatformStrings)(nil)
+
 // HasExt returns whether this set contains a file with the given extension.
 func (ps *PlatformStrings) HasExt(ext string) bool {
 	return ps.firstExtFile(ext) != ""

rule/value.go

@@ -37,6 +37,8 @@ type GlobValue struct {
 	Excludes []string
 }
 
+var _ BzlExprValue = (*GlobValue)(nil)
+
 func (g GlobValue) BzlExpr() bzl.Expr {
 	patternsValue := ExprFromValue(g.Patterns)
 	globArgs := []bzl.Expr{patternsValue}
@@ -72,6 +74,9 @@ type Merger interface {
 
 type SortedStrings []string
 
+var _ BzlExprValue = SortedStrings(nil)
+var _ Merger = SortedStrings(nil)
+
 func (s SortedStrings) BzlExpr() bzl.Expr {
 	list := make([]bzl.Expr, len(s))
 	for i, v := range s {
@@ -93,6 +98,8 @@ func (s SortedStrings) Merge(other bzl.Expr) bzl.Expr {
 
 type UnsortedStrings []string
 
+var _ Merger = UnsortedStrings(nil)
+
 func (s UnsortedStrings) Merge(other bzl.Expr) bzl.Expr {
 	if other == nil {
 		return ExprFromValue(s)
@@ -104,6 +111,8 @@ func (s UnsortedStrings) Merge(other bzl.Expr) bzl.Expr {
 // select expression that picks a string list based on a string condition.
 type SelectStringListValue map[string][]string
 
+var _ BzlExprValue = SelectStringListValue(nil)
+
 func (s SelectStringListValue) BzlExpr() bzl.Expr {
 	defaultKey := "//conditions:default"
 	keys := make([]string, 0, len(s))