Issue #3075785 by alexpott, andypost, mondrake, Mixologic, greg.1.anderson: Update composer/composer to ^1.9.1

Issue #3091225 by alexpott, mondrake: Require-dev mikey179/vfsstream ^1.6.8 in order to support PHP 7.4

Issue #3087482 by Mile23, greg.1.anderson, alexpott, xjm, AaronMcHale, johndevman, mbaynton, Mixologic, drumm: Add 'next steps' info to Composer installation output

Issue #3088403 by Mile23, Mixologic: Defensive coding: Vendor hardening plugin sometimes receives AliasPackage

(cherry picked from commit 3072f841)

Issue #3090684 by Mixologic, greg.1.anderson, opdavies, alexpott: Rename drupal/dev-dependencies and drupal/pinned-dev-dependencies

(cherry picked from commit 7d71991c)

Issue #3087626 by greg.1.anderson, Mixologic, jibran, Mile23, alexpott, xjm: Convert drupal/core-recommended & c. into a subtree split

Issue #3082866 by Mile23, Mixologic, greg.1.anderson, alexpott, rfay: composer complains "Skipped installation of bin bin/composer for package composer/composer: file not found in package"

(cherry picked from commit dd271560)

Issue #3082866 by Mile23, Mixologic, greg.1.anderson, alexpott, rfay: composer complains "Skipped installation of bin bin/composer for package composer/composer: file not found in package"

Issue #3085697 by greg.1.anderson, Mixologic, mbaynton, larowlan: Allow scaffold plugin to append to non-scaffolded files

Revert "Issue #3085697 by greg.1.anderson, mbaynton, Mixologic, larowlan: Allow scaffold plugin to append to non-scaffolded files"

This reverts commit c5ac07aa.

Issue #3085697 by greg.1.anderson, mbaynton, Mixologic, larowlan: Allow scaffold plugin to append to non-scaffolded files

Issue #3086148 by greg.1.anderson, hussainweb, Mixologic: Make scaffold "extra" data key conform with plugin command name

Issue #3085354 by greg.1.anderson, mbaynton, hussainweb: Better default behavior for scaffold-created .gitignore files

Issue #3085608 by Wim Leers, mikelutz, voleger, larowlan: [Symfony 4] Symfony console command ::execute() methods should return 0 on success

Issue #3085075 by greg.1.anderson: Add '-s dev' to Composer Project Template documentation so instructions work w/out a stable release

Issue #3084326 by hussainweb, greg.1.anderson: Install .editorconfig and .gitattributes at the project root

Issue #3084298 by greg.1.anderson: Make composer installer paths consistent between drupal/drupal and the project templates

Issue #2982680 by greg.1.anderson, mbaynton, Mixologic, jibran, webflo, Mile23, phenaproxima, AaronMcHale, Charlie ChX Negyesi, nicksanta: Add composer-ready project templates to Drupal core

Issue #3080205 by greg.1.anderson: Implicitly allow drupal/core and legacy scaffold projects to scaffold files

Issue #3080649 by greg.1.anderson, larowlan, BinaryBlock, alexpott, danielnv18, Mile23: Drupal core scaffolding : path to vendor directory in generated autoload.php file is incorrect

Issue #3079481 by greg.1.anderson, Mixologic, larowlan, Mile23, Charlie ChX Negyesi, catch, Jaesin, alexpott: Fix problems Vendor Hardening Plugin has with using the File Security component

Issue #3057094 by Mile23, hussainweb, greg.1.anderson, Mixologic, alexpott, catch, bojanz, andypost, leolando.tan: Add Composer vendor/ hardening plugin to core