[5.x] Merge config field defaults into field config

[duncanmcclean]

This pull request fixes #10106, where the Code Fieldtype wouldn't retain the selected mode even when the mode_selectable config option looked like it was enabled (by default, it's set to true).

This was happening since the Code::process() method saves just the code value and not the mode/language when mode_selectable is false:

cms/src/Fieldtypes/Code.php

Lines 152 to 154 in 73c48a4

	if (! $this->isModeSelectable()) {
	return $value['code'];
	}

cms/src/Fieldtypes/Code.php

Lines 194 to 197 in 73c48a4

	private function isModeSelectable()
	{
	return $this->config('mode_selectable', false);
	}

This PR fixes that by ensuring the default values for fieldtype config fields are included in the output of Fieldtype::config().

Fixes #10106.

[jasonvarga]

I'm gonna split this PR into two. The other half is in #10246.

[jasonvarga]

I don't consider the "default" changes (like automatic line breaks in markdown) breaking as we expected them as of 5.0.0. I would consider us not changing them to be bugs.

Plus, in 4.x you probably would have the defaults saved to your yaml file (which in automatic line break's case was true), and not even change anyway.