chore(build): Remove prepare script; run deps script from start script (#6653)

To make `npm install` and `npm ci` faster, and to avoid redundant
work when doing either of these followed by `npm run build` (or
any other script which does a build), delete the `prepare`
script and instead add an invocation of `npm run deps` to the
beginning of the `start` script.

gulpfile.js

@@ -41,7 +41,6 @@ module.exports = {
   gitUpdateGithubPages: gitTasks.updateGithubPages,
 
   // Manually-invokable targets, with prequisites where required.
-  prepare: buildTasks.prepare,
   format: buildTasks.format,
   messages: buildTasks.messages,  // Generate msg/json/en.json et al.
   sortRequires: cleanupTasks.sortRequires,

package.json

@@ -44,13 +44,12 @@
     "minify": "gulp minify",
     "package": "gulp package",
     "postinstall": "patch-package",
-    "prepare": "gulp prepare",
     "prepareDemos": "gulp prepareDemos",
     "publish": "npm ci && gulp publish",
     "publish:beta": "npm ci && gulp publishBeta",
     "recompile": "gulp recompile",
     "release": "gulp gitCreateRC",
-    "start": "concurrently -n tsc,server \"tsc --watch --preserveWatchOutput --outDir 'build/src' --declarationDir 'build/declarations'\" \"http-server ./ -s -o /tests/playground.html -c-1\"",
+    "start": "npm run deps && concurrently -n tsc,server \"tsc --watch --preserveWatchOutput --outDir 'build/src' --declarationDir 'build/declarations'\" \"http-server ./ -s -o /tests/playground.html -c-1\"",
     "tsc": "gulp tsc",
     "test": "gulp test",
     "test:generators": "gulp testGenerators",

scripts/gulpfiles/build_tasks.js

@@ -288,25 +288,6 @@ const JSCOMP_OFF = [
   'visibility',
 ];
 
-/**
- * The npm prepare script, run by npm install after installing modules
- * and as part of the packaging process.
- *
- * This does just enough of the build that npm start should work.
- *
- * Exception: when running in the CI environment, we don't build
- * anything.  We don't need to, because npm test will build everything
- * needed, and we don't want to, because a tsc error would prevent
- * other workflows (like lint and format) from completing.
- */
-function prepare(done) {
-  if (process.env.CI) {
-    done();
-    return;
-  }
-  return exports.deps(done);
-}
-
 /**
  * Builds Blockly as a JS program, by running tsc on all the files in
  * the core directory.
@@ -760,7 +741,6 @@ exports.minify = gulp.series(exports.deps, buildCompiled);
 exports.build = gulp.parallel(exports.minify, exports.langfiles);
 
 // Manually-invokable targets, with prequisites where required.
-exports.prepare = prepare;
 exports.format = format;
 exports.messages = generateMessages;  // Generate msg/json/en.json et al.
 exports.buildAdvancedCompilationTest =