Enable ifdef-loader preprocessing with webpack 7+

@ngtools+webpack+9.1.10.patch

Since @ngtools/webpack doesn't support preprocess chain loader, see (here)[https://github.com/nippur72/ifdef-loader/issues/4].
We use process.env to pass the ifdef loader options and patch the compiler (webpack) to read the env and parse the files.
We use a prefix 'IFDEF_' to filter the environment variables.

This is more a fix than anything else for now. A better fix would have been a PR to enable webpack to use any preprocessor 
but I don't have the time right now.

diff --git a/node_modules/@ngtools/webpack/src/compiler_host.js b/node_modules/@ngtools/webpack/src/compiler_host.js
index 58d942c..a7db0be 100644
--- a/node_modules/@ngtools/webpack/src/compiler_host.js
+++ b/node_modules/@ngtools/webpack/src/compiler_host.js
@@ -11,10 +11,35 @@ const core_1 = require("@angular-devkit/core");
 const fs_1 = require("fs");
 const ts = require("typescript");
 const utils_1 = require("./utils");
+const { parse } = require("ifdef-loader/preprocessor");
+const dotenvParseVariables = require('dotenv-parse-variables');
 const dev = Math.floor(Math.random() * 10000);
+
+
+class IfdefLoaderOptions {
+    constructor(options){
+        this._options = options;
+    }
+
+    get(){
+        return this._options;
+    }
+
+    static fromProcessEnv(parsedEnv, prefix='IFDEF_'){
+        let options = {}
+        for(let k in parsedEnv){
+            if(k.startsWith(prefix)){
+                options[k.replace(prefix, '')] = parsedEnv[k]
+            }
+        }
+        return new IfdefLoaderOptions(options)
+    }
+}
+
 class WebpackCompilerHost {
     constructor(_options, basePath, host, cacheSourceFiles, directTemplateLoading = false, ngccProcessor, moduleResolutionCache) {
         this._options = _options;
+        this._ifdefLoaderOptions = IfdefLoaderOptions.fromProcessEnv(dotenvParseVariables(process.env)).get()
         this.cacheSourceFiles = cacheSourceFiles;
         this.directTemplateLoading = directTemplateLoading;
         this.ngccProcessor = ngccProcessor;
@@ -258,8 +283,10 @@ class WebpackCompilerHost {
             if (cached) {
                 return cached;
             }
-            const content = this.readFile(fileName);
+
+            let content = this.readFile(fileName);
             if (content !== undefined) {
+                if(this._ifdefLoaderOptions) content = parse(content, this._ifdefLoaderOptions, this._ifdefLoaderOptions['ifdef-verbose'] ? this._ifdefLoaderOptions['ifdef-verbose'] : true, this._ifdefLoaderOptions['ifdef-triple-slash'] ? this._ifdefLoaderOptions['ifdef-triple-slash'] : true, fileName.replace(`${this.getCurrentDirectory()}/`, ''))
                 const sf = ts.createSourceFile(utils_1.workaroundResolve(fileName), content, languageVersion, true);
                 if (this.cacheSourceFiles) {
                     this._sourceFileCache.set(p, sf);

custom-webpack.config.js

/* 
  We pass the options to ifdef-loader using the process.env variables.
  We use a prefix 'IFDEF_' to differentiate the ones for ifdef-loader 
  (must be the same as the one used by the webpack compiler).
*/

let opts = {
  RUN_ENV: process.env.RUN_ENV,
  ENABLE_XXX: process.env.ENABLE_XXX
};

for(let k in opts) process.env[`IFDEF_${k.trim()}`] = opts[k];

package.json

/* 
  The patch is generated & applied using patch-package.
*/

{
  "scripts": {
    "postinstall": "patch-package"
  },
  "devDependencies": {
    "patch-package": "^6.2.2",
    "dotenv-parse-variables": "^1.0.1"
  }
}