fix(json-schema): add missing `platforms` property to cmds `for`

[dkarter]

Motivation

I've been getting errors from my editor that using platforms as a sibling of for under cmds is not supported, when it is in fact supported and respected by task

I believe this was missed on #980

Changes

• This change simply adds a small fix to the json schema to denote that platforms is allowed with for

Screenshots / Testing Instructions

🖼️ Before

🖼️ After

[dkarter]

@vmaerten great suggestion! Fixed in 4065c3e

[vmaerten]

Thanks