Ensure compatibility with Terser pure_getters optimization

[earshinov]

Hello!

This change makes Quill source compatible with Terser pure_getters optimization.

This change is needed for Angular projects, because:

1. Angular CLI (the most popular tool for building Angular projects) internally uses Webpack with terser-webpack-plugin and unconditionally applies pure_getters optimization. You can see all of it in Angular CLI sources: webpack-configs/common.ts.

   At some point Angular CLI team disabled pure_getters optimization (see Production build optimization breaks code angular/angular-cli#11439, fix(@angular-devkit/build-angular): remove pure_getters angular/angular-cli#14187), but then they returned it back for their reasons (Revert "fix(@angular-devkit/build-angular): remove pure_getters" angular/angular-cli#14287).

2. Angular CLI not only compiles project code, but also recompiles the code of external libraries located in node_modules, including quill.min.js if it happens to be there

Here is the problematic code from Quill:

function contains(parent, descendant) {
  try {
    // Firefox inserts inaccessible nodes around video elements
    descendant.parentNode; // eslint-disable-line no-unused-expressions
  } catch (e) {
    return false;
  }
  return parent.contains(descendant);
}

With pure_getters optimization, Terser duly assumes that descendant.parentNode statement is not doing anything and prunes the whole try..catch block. You can easily reproduce the problem locally:

$ terser -c pure_getters <<EOF
export function contains(parent, descendant) {
  try {
    // Firefox inserts inaccessible nodes around video elements
    descendant.parentNode; // eslint-disable-line no-unused-expressions
  } catch (e) {
    return false;
  }
  return parent.contains(descendant);
}
EOF
export function contains(parent,descendant){return parent.contains(descendant)}

Of course, the resulting code will throw the Error described in #644 because the guards aren't there.