Fix missing abstract keyword in completion (#45710)

* first commit

* tests

src/harness/fourslashInterfaceImpl.ts

@@ -1292,6 +1292,7 @@ namespace FourSlashInterface {
             "let",
             "package",
             "yield",
+            "abstract",
             "as",
             "asserts",
             "any",
@@ -1495,6 +1496,7 @@ namespace FourSlashInterface {
             "let",
             "package",
             "yield",
+            "abstract",
             "as",
             "asserts",
             "any",

src/services/completions.ts

@@ -2960,6 +2960,7 @@ namespace ts.Completions {
                         || kind === SyntaxKind.ModuleKeyword
                         || kind === SyntaxKind.TypeKeyword
                         || kind === SyntaxKind.NamespaceKeyword
+                        || kind === SyntaxKind.AbstractKeyword
                         || isTypeKeyword(kind) && kind !== SyntaxKind.UndefinedKeyword;
                 case KeywordCompletionFilters.FunctionLikeBodyKeywords:
                     return isFunctionLikeBodyKeyword(kind);