feat: downlevel .d.ts for older TS versions (#320)

allows to use spectator 5.x with Angular 8 with TS 3.5

Signed-off-by: Andrey Chalkin <L2jLiga@gmail.com>

package.json

@@ -5,7 +5,7 @@
   "scripts": {
     "postinstall": "ngcc",
     "ng": "ng",
-    "build": "ng build --prod && npm run build:schematics && npm run copy:schematics && npm run copy:docs && npm run copy:bin",
+    "build": "ng build --prod && npm run build:schematics && npm run copy:schematics && npm run copy:docs && npm run copy:bin && npm run downlevel-dts",
     "build:schematics": "tsc -p projects/spectator/schematics/tsconfig.json",
     "test": "ng test",
     "test:jest": "ng run spectator:test-jest",
@@ -20,6 +20,7 @@
     "copy:bin": "cp migrate.js dist/spectator",
     "copy:docs": "cp *.md dist/spectator",
     "copy:schematics": "cp -r projects/spectator/schematics/src/ dist/spectator/schematics",
+    "downlevel-dts": "downlevel-dts ./dist/spectator ./dist/spectator/ts3.4",
     "postbump": "npm run build",
     "release": "cd projects/spectator && standard-version --infile ../../CHANGELOG.md"
   },
@@ -30,7 +31,6 @@
     "tslib": "^1.10.0"
   },
   "devDependencies": {
-    "git-cz": "^3.2.1",
     "@angular-builders/jest": "^8.0.4",
     "@angular-devkit/build-angular": "~0.900.1",
     "@angular-devkit/build-ng-packagr": "~0.900.1",
@@ -57,6 +57,8 @@
     "core-js": "^2.5.4",
     "cross-env": "^5.1.4",
     "cz-conventional-changelog": "^2.1.0",
+    "downlevel-dts": "^0.5.0",
+    "git-cz": "^3.2.1",
     "helpful-decorators": "^1.7.2",
     "husky": "^0.14.3",
     "jasmine-core": "~3.5.0",

projects/spectator/package.json

@@ -36,5 +36,8 @@
   "bin": {
     "spectator-migrate": "./migrate.js"
   },
-  "schematics": "./schematics/collection.json"
+  "schematics": "./schematics/collection.json",
+  "typesVersions": {
+    "<3.7": { "*": ["ts3.4/*"] }
+  }
 }