feat!: set SystemRequirements field automatically

[eitsupi]

Currently a message to notice this field (Please update the system requirement in 'DESCRIPTION' file.), but I do not think there is any disadvantage to setting it up automatically.

The notation SystemRequirements: Cargo (rustc package manager) has been used in gifski, hellorust etc.
https://github.com/cran/gifski/blob/8bc440a1e3921af1f37a0deb27a01b8274d9b138/DESCRIPTION#L16
https://github.com/cran/hellorust/blob/2265950f01f2bbd87bab26c4f242b9fa8c9817dc/DESCRIPTION#L17

[CGMossa]

Another great idea. However, this is a "cran" / free field, and could contain
a number of things. Some packages include that you need GNUMake to compile a package.

[eitsupi]

However, this is a "cran" / free field, and could contain
a number of things. Some packages include that you need GNUMake to compile a package.

Of course, I understand that and therefore will not overwrite it if SystemRequirements has already been set. This is as confirmed by the test.

[eitsupi]

Is there any reason this can't be merged?

[Ilia-Kosenkov]

LGTM, should I merge right now or do you have anything else to modify?

[eitsupi]

Thank you for your review. I think you can merge this one right away.