spec: add new language for alias declarations
spec: add new language for alias declarations

For #16339.

Change-Id: I7d912ea634bbfacfc0217f97dccb270fde06f16b
Reviewed-on: https://go-review.googlesource.com/30601
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

akutz/go,goodlang/good,josharian/go,Cofyc/go,josharian/go,vsekhar/elastic-go,josharian/go,polyverse-security/go,cloudflare/go,josharian/go,nawawi/go,CAFxX/go,cloudflare/go,polyverse-security/go,CAFxX/go,linux-on-ibm-z/go,nawawi/go,josharian/go,christopher-henderson/Go,derekmarcotte/go,polyverse-security/go,akutz/go,nawawi/go,akutz/go,polyverse-security/go,nawawi/go,suncycheng/go,vsekhar/elastic-go,linux-on-ibm-z/go,CAFxX/go,Cofyc/go,Cofyc/go,linux-on-ibm-z/go,vsekhar/elastic-go,ImJasonH/go,linux-on-ibm-z/go,Cofyc/go,goodlang/good,suncycheng/go,golang/go,CAFxX/go,golang/go,linux-on-ibm-z/go,CAFxX/go,golang/go,Cofyc/go,derekmarcotte/go,golang/go,cloudflare/go,ImJasonH/go,goodlang/good,linux-on-ibm-z/go,derekmarcotte/go,vsekhar/elastic-go,cloudflare/go,CAFxX/go,CAFxX/go,vsekhar/elastic-go,polyverse-security/go,polyverse-security/go,akutz/go,suncycheng/go,cloudflare/go,goodlang/good,CAFxX/go,derekmarcotte/go,vsekhar/elastic-go,suncycheng/go,nawawi/go,codestation/go,codestation/go,ImJasonH/go,golang/go,goodlang/good,goodlang/good,ImJasonH/go,Cofyc/go,Cofyc/go,cloudflare/go,codestation/go,derekmarcotte/go,akutz/go,nawawi/go,suncycheng/go,linux-on-ibm-z/go,akutz/go,polyverse-security/go,ImJasonH/go,akutz/go,cloudflare/go,codestation/go,suncycheng/go,josharian/go,derekmarcotte/go,golang/go,codestation/go,linux-on-ibm-z/go,polyverse-security/go,derekmarcotte/go,akutz/go,suncycheng/go,goodlang/good,ImJasonH/go,derekmarcotte/go,golang/go,codestation/go,Cofyc/go,josharian/go,cloudflare/go,nawawi/go,suncycheng/go,golang/go,codestation/go,vsekhar/elastic-go,ImJasonH/go,ImJasonH/go,vsekhar/elastic-go,nawawi/go,codestation/go,goodlang/good,josharian/go