Support new_issue in autolabel config

[Noratrieb]

Just like new_pr, it adds a label to every new issue that is opened.

See https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/make.20untriaged.20issues.20explicit for the motivation.

I've tested it on https://github.com/Nilstrieb/some-cursed-repo, it seems to behave correctly for what I tested.
I tested

[autolabel."wontfix"]
trigger_labels = [
  "bug"
]

[autolabel."invalid"]
new_issue = true

[autolabel."documentation"]
new_pr = true

[autolabel."help wanted"]
new_pr = true
new_issue = true
exclude_labels = ["duplicate"]

• create an issue, "help wanted" and "invalid" are assigned
• create an issue with the "duplicate" label, only "invalid" is assigned
• create a PR, "help wanted" and "documentation" are assigned
• create a PR with the "duplicate" label, only "documentation" is assigned
• add "bug" to an issue, "wontfix" is assigned (this is very useful)
• add "bug" to a PR "wontfix" is assigned

documentation: rust-lang/rust-forge#690