Add more types and documentation to protocol file

Reduce the chances that someone reading this file forgets the intent
of the extensible 'enum' namespaces by redundantly putting it loudly
at the beginning.

Add some eponymous type aliases for these 'enum' types so that code
can refer to them.

Author: jcreedcmu

extensions/ql-vscode/src/messages.ts

@@ -1,6 +1,17 @@
 /**
  * Types for messages exchanged during jsonrpc communication with the
  * the CodeQL query server.
+ *
+ * This file exists in the queryserver and in the vscode extension, and
+ * should be kept in sync between them.
+ *
+ * A note about the namespaces below, which look like they are
+ * essentially enums, namely Severity, ResultColumnKind, and
+ * QueryResultType. By design, for the sake of extensibility, clients
+ * receiving messages of this protocol are supposed to accept any
+ * number for any of these types. We commit to the given meaning of
+ * the numbers listed in constants in the namespaces, and we commit to
+ * the fact that any unknown QueryResultType value counts as an error.
  */
 
 import * as rpc from 'vscode-jsonrpc';
@@ -294,10 +305,13 @@ export interface CompilationMessage {
   /**
    * The severity of the message
    */
-  severity: number;
+  severity: Severity;
 }
+
+export type Severity = number;
 /**
- * Severity of different messages
+ * Severity of different messages. This namespace is intentionally not
+ * an enum, see "for the sake of extensibility" comment above.
  */
 export namespace Severity {
   /**
@@ -333,16 +347,18 @@ export interface ResultColumn {
    * The kind of the column. See `ResultColumnKind`
    * for the current possible meanings
    */
-  kind: number;
+  kind: ResultColumnKind;
   /**
    * The name of the column.
    * This may be compiler generated for complex select expressions.
    */
   name: string;
 }
 
+export type ResultColumnKind = number;
 /**
- * The kind of a result column.
+ * The kind of a result column. This namespace is intentionally not an enum, see "for the sake of
+ * extensibility" comment above.
  */
 export namespace ResultColumnKind {
   /**
@@ -748,7 +764,7 @@ export interface EvaluationResult {
    * The type of the result. See QueryResultType for
    * possible meanings. Any other result should be interpreted as an error.
    */
-  resultType: number;
+  resultType: QueryResultType;
   /**
    * The wall clock time it took to evaluate the query.
    * The time is from when we initially tried to evaluate the query
@@ -762,8 +778,10 @@ export interface EvaluationResult {
   message?: string;
 }
 
+export type QueryResultType = number;
 /**
- * The result of running a query,
+ * The result of running a query. This namespace is intentionally not
+ * an enum, see "for the sake of extensibility" comment above.
  */
 export namespace QueryResultType {
   /**
@@ -818,7 +836,7 @@ export interface RunUpgradeResult {
    * The type of the result. See QueryResultType for
    * possible meanings. Any other result should be interpreted as an error.
    */
-  resultType: number;
+  resultType: QueryResultType;
   /**
    * The error message if an error occurred
    */