types/sql-bricks-postgres: Add initial typings

This commit declares and type-checks the following clauses:

* limit().offset() chains on SelectStatement
* onConflict().doUpdate() chains on InsertStatement

We need to flesh out the other possibilities for onConflict() in
particular before all these changes get submitted as an upstream
PR.

types/sql-bricks-postgres/index.d.ts

@@ -0,0 +1,30 @@
+declare module "sql-bricks-postgres" {
+  import * as sql from "sql-bricks";
+  export * from "sql-bricks";
+
+  interface OnConflictClause {
+    doUpdate(colNames: string[]): sql.Statement;
+  }
+
+  interface LimitClause extends sql.SelectStatement {
+    offset(value: number): sql.Statement;
+  }
+
+  export interface PgInsertStatement extends sql.InsertStatement {
+    onConflict(...colNames: string[]): OnConflictClause;
+  }
+
+  export interface PgSelectStatement extends sql.SelectStatement {
+    limit(value: number): LimitClause;
+  }
+
+  export interface PgSqlBricksFn extends sql.SqlBricksFn {
+    insert(tbl?: string, ...values: any[]): PgInsertStatement;
+
+    select(...columns: Array<string | sql.SelectStatement>): PgSelectStatement;
+    select(columns: string[] | sql.SelectStatement[]): PgSelectStatement;
+  }
+
+  const PgSqlBricks: PgSqlBricksFn;
+  export default PgSqlBricks;
+}