Finished adding basic category adding functionality

OpenArchival.Database/TablesConstants.cs

@@ -0,0 +1,12 @@
+namespace OpenArchival.Database;
+
+public static class TablesConstants
+{
+    public static string CreateCategoriesTable = """
+        CREATE TABLE IF NOT EXISTS Categories (
+            categoryname TEXT NOT NULL PRIMARY KEY,
+            fieldseperator TEXT NOT NULL,
+            fieldnames TEXT [] NOT NULL
+        );
+        """;
+}