Migrate Layout Basics Codelab to Null Safety (#5227)

* copy of layout-basics.md codelab

* imported gist codelab into markdown

* migrated example

* copy dartpad gist to embedded

* migrated example to null safety

* copy dartpad gist to embedded

* migrated mainAxisAlignment property

* copy crossAxisAlignment property from gist to markdown

* migrated crossAxisAlignment property exercise

* remove comment

* copy Flexible widget gist to markdown

* migrated Flexible Widget exercise

* copy Flexible widget 2 gist to markdown

* migrated Flexible widget part 2

* copy Expanded widget gist to markdown

* migrated Expanded widget to null safety

* copy SizedBox widget gist to markdown

* migrated SizedBox to null safety

* copy and migrate SizedBox 2

* copy Spacer gist to markdown

* migrated Spacer to null safety

* copy Text widget gist to markdown

* migrated Text widget to null safety

* copy Icon widget gist to markdown

* migrated Icon widget to null safety

* copy Image widget gist to markdown

* migrated Image widget to null safety

* copy Putting it all together from gist to markdown

* Migraged Part 1.1 to null safety

- deprecated text styles replaced (body and headline)

* migrated Part 1.2 to null safety

* migrated part 2.1

* migrated part 2.2

* migrated part 3

* remove run-true from dartpads

* Adjust indentation in Putting it all together exercises

* skip checks on dartpad code

Co-authored-by: Miquel Beltran <miquelbeltran@users.noreply.github.com>

Author: miquelbeltran