Add inline node inclusion support to mod.yaml.

[pchote]

This PR implements my take on #18277. This allows multiple Include: nodes to be defined, inserting the contents of the referenced file at the node's location.

This allows for an IMO more natural syntax:

Metadata:
	...etc...

Include: rules/manifest.yaml
Include: sequences/manifest.yaml
Include: tilesets/manifest.yaml

as well as include/overrides:

Include: ../common/fonts.yaml
Fonts:
	Title:
		Font: ra|ZoodRangmah.ttf
		Size: 48
		Ascender: 26

(although IMO this would be bad practice because ../common/* won't work for SDK based mods with multiple mod roots)

[reaperrr]

https://github.com/OpenRA/OpenRA/wiki/Changelog-(bleed)/_compare/97d32de2ab2fb33b66813c47aad58481b7a1e4f3...b74946e4f494b55db3d7a6a888ffa0b4f008ffab