Type-only imports and exports (#35200)

* Add type-only support for export declarations

* Use a synthetic type alias instead of binding type-only exports as a type alias

* Works for re-exports!

* isolatedModules works fine

* Diagnostic for type-only exporting a value

* Start isolated modules codefix

* Update for LKG control flow changes

* Type-only import clause parsing

* Type-only default import checking

* Type-only named imports

* Fix isolated modules error

* Filter namespaces down to type-only

* Fix class references

* Test nested namespaces

* Test circular type-only imports/exports

* Fix getTypeAtLocation for type-only import/export specifiers

* Fix type-only generic imports

* Update public APIs

* Remove unused WIP comment

* Type-only namespace imports

* Fix factory update calls

* Add grammar errors for JS usage and mixing default and named bindings

* Update updateExportDeclaration API baseline

* Fix grammar checking import clauses

* Enums, sort of

* Dedicated error for type-only enum

* Skip past type-only alias symbols in quick info

* Update error code in baseline

* WIP: convertToTypeOnlyExport

* isolatedModules codefix (single export declaration)

* isolatedModules code fix (all)

* Stop eliding non-type-only imports by default, add compiler flag

* Update to match updated diagnostic messages

* Update more baselines

* Update more tests

* Auto-import as type-only

* Add codefix for splitting type-only import with default and named bindings

* Add more services tests

* Add targeted error message for "export type T;" when T exists

* Add targeted error for "import type T = require(...)"

* Flip emit flag

* Add test for preserveUnusedImports option

* Fix flag flip on import =

* Make compiler option string-valued

* Fix merge conflicts

* Add --importsNotUsedAsValue=error

* Phrasing of messages.

Co-authored-by: Daniel Rosenwasser <DanielRosenwasser@users.noreply.github.com>

src/compiler/commandLineParser.ts

@@ -472,6 +472,17 @@ namespace ts {
             category: Diagnostics.Basic_Options,
             description: Diagnostics.Import_emit_helpers_from_tslib
         },
+        {
+            name: "importsNotUsedAsValue",
+            type: createMapFromTemplate({
+                remove: ImportsNotUsedAsValue.Remove,
+                preserve: ImportsNotUsedAsValue.Preserve,
+                error: ImportsNotUsedAsValue.Error
+            }),
+            affectsEmit: true,
+            category: Diagnostics.Advanced_Options,
+            description: Diagnostics.Specify_emit_Slashchecking_behavior_for_imports_that_are_only_used_for_types
+        },
         {
             name: "downlevelIteration",
             type: "boolean",