add routes for block, validate block slug

internal/block/block.go

@@ -2,12 +2,24 @@ package block
 
 import (
 	"encoding/json"
+	"fmt"
+	"regexp"
 )
 
+var validSlug = regexp.MustCompile(`^[a-z0-9]+$`)
+
 type Block struct {
+	Slug       string
 	Components []Component
 }
 
+func NewBlock(slug string, components []Component) (*Block, error) {
+	if !validSlug.MatchString(slug) {
+		return nil, fmt.Errorf("invalid block slug %q: must be a-z0-9 only", slug)
+	}
+	return &Block{Slug: slug, Components: components}, nil
+}
+
 type BlockInstance struct {
 	Components []ComponentInstance
 }