Add as completions (microsoft#36359)

Author: sandersn

src/harness/fourslashInterfaceImpl.ts

@@ -1152,6 +1152,7 @@ namespace FourSlashInterface {
             "let",
             "package",
             "yield",
+            "as",
             "asserts",
             "any",
             "async",
@@ -1352,6 +1353,7 @@ namespace FourSlashInterface {
             "let",
             "package",
             "yield",
+            "as",
             "asserts",
             "any",
             "async",

src/services/completions.ts

@@ -2461,6 +2461,7 @@ namespace ts.Completions {
                         || kind === SyntaxKind.ModuleKeyword
                         || kind === SyntaxKind.TypeKeyword
                         || kind === SyntaxKind.NamespaceKeyword
+                        || kind === SyntaxKind.AsKeyword
                         || isTypeKeyword(kind) && kind !== SyntaxKind.UndefinedKeyword;
                 case KeywordCompletionFilters.FunctionLikeBodyKeywords:
                     return isFunctionLikeBodyKeyword(kind);

tests/cases/fourslash/completionTypeAssertion.ts

@@ -0,0 +1,5 @@
+/// <reference path="fourslash.ts" />
+
+//// var x = this as/*1*/
+
+verify.completions({marker: "1", exact: completion.globalsPlus(["x"]) })