Add basic Swift support

The extension doesn't actually use anything regarding the language of
variant analyses, so this just updates some types.

The actual Swift support is done in the CLI, which is also used for
determining which languages are actually supported. So, the environment
variable is already used by the CLI for showing supported languages.

extensions/ql-vscode/src/helpers.ts

@@ -551,6 +551,7 @@ export const dbSchemeToLanguage = {
   "semmlecode.csharp.dbscheme": "csharp",
   "go.dbscheme": "go",
   "ruby.dbscheme": "ruby",
+  "swift.dbscheme": "swift",
 };
 
 export const languageToDbScheme = Object.entries(dbSchemeToLanguage).reduce(

extensions/ql-vscode/src/remote-queries/gh-api/variant-analysis.ts

@@ -16,7 +16,8 @@ export type VariantAnalysisQueryLanguage =
   | "java"
   | "javascript"
   | "python"
-  | "ruby";
+  | "ruby"
+  | "swift";
 
 export interface VariantAnalysis {
   id: number;

extensions/ql-vscode/src/remote-queries/shared/variant-analysis.ts

@@ -34,6 +34,7 @@ export enum VariantAnalysisQueryLanguage {
   Javascript = "javascript",
   Python = "python",
   Ruby = "ruby",
+  Swift = "swift",
 }
 
 export function parseVariantAnalysisQueryLanguage(