refactor(build): Preparation for building TypeScript (#6205)

* chore(deps): Update closure/goog/base.js, add goog.js

  - Update base.js from the latest version (20220502.0.0).
  - Also copy over goog.js, which provides access to a suitable subset
    of goog.* via an importable module).

* chore(build): Split gulpfiles/config.js exports object

  This makes it possible for entries to depend on each other.

* chore(build): build config consistency

  - Reorder entries in gulpfiles.config.js to better match order they
    are used.
  - Have update_metadata.sh reference config.js and vice versa.

* refactor(build): Move deps.js (+ deps.mocha.js) from test/ to build/

  Once we start using tsc, deps.js will be created based on the ouptut
  of tsc rather than the raw source in core/.  Since tsc will need to
  be run before running closure-make-deps and also before trying to
  load blockly_uncompressed.js, it doesn't really make sense to check
  in deps.js; it's better to re-create as needed.

  To reduce inconvenience, a new "prepare" script is added to
  package.json which will run the buildDeps gulp target automaticaly
  when one runs npm install.

* refactor(build): Always build from TypeScript sources

  - Add buildJavaScript gulp task to use tsc to compile any .ts files
    in core/ into build/src/core/ (and also copy any .js files that
    are not yet migrated to TypeScript, which for now is all of them.
  - Remove closure/goog from explicit inputs to tsc; it will find
    the files it needs (e.g., goog.js) automatically.
  - Have buildDeps, the playground, and all the tests that run in
    uncompiled mode use build/src/core/ instead of core/ as their
    input directory.

* feat(build): Add buildJavaScriptAndDeps gulp task

  Have npm run build:deps (and npm run prepare) use a new gulp task,
  buildJavaScriptAndDeps, to run tsc followed by closure-make-deps,
  ensuring that deps.js is calculated based on the most recent code
  in core/.

* fix(build): Fix implementation of flattenCorePaths

  Even though this function is going away I want to remove it in
  a separate PR so that we can revert easily if desired.  But the
  previous checked-in code was totally wrong.  This version works.

* fix(build): Don't let checkinBuilt copy build/src/**

  Now that we are putting a lot more stuff in build/ (specifically,
  all the tsc output in build/src/), modify checkinBuilt so that it
  only copies the specific things we want to check in (for now):

  - _compressed.js build artifacts and their accompanying .js.maps
  - the generated build/msg/js/*.js language files.

  Unrelatedly, also fix safety-quoting of arguments for one execSync
  call.

tsconfig.json

@@ -1,7 +1,7 @@
 {
   "include": [
-    "core/**/*",
-    "closure/goog/base_minimal.js",
+    "core/**/*",  // N.B.: also pulls in closure/goog/goog.js if needed.
+    "closure/goog/goog.js",  // Just for ouptut directory structure.
   ],
   "compilerOptions": {
     // Tells TypeScript to read JS files, as
@@ -16,7 +16,6 @@
     // next to the .js files
     //"declarationDir": "build/ts/declarations",
 
-    "outDir": "build/ts",
     "module": "ES2015",
     "moduleResolution": "node",
     "target": "ES2020",