[dev.typeparams] cmd/compile/internal/syntax: prepare syntax nodes for type parameters

- add TParamList fields to TypeDecl, FuncDecl
- also: change File.Lines to File.EOF so we have the actual file end position

Change-Id: Ia345f888080a884f7ac5cefd8bff3d80e4a59cdc
Reviewed-on: https://go-review.googlesource.com/c/go/+/261657


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>