Do a large chunk of the move to go

This commit moves a huge chunk of the code to go. The TODO.md document
indicates what is left to be done.

env/requirements.go

@@ -0,0 +1,24 @@
+package env
+
+import (
+	"github.com/tkw1536/goprogram"
+	"github.com/tkw1536/goprogram/meta"
+)
+
+type Requirements struct {
+	NeedsConfig bool
+}
+
+// AllowsFlag checks if the provided flag may be passed to fullfill this requirement
+// By default it is used only for help page generation, and may be inaccurate.
+func (r Requirements) AllowsFlag(flag meta.Flag) bool {
+	return true
+}
+
+// Validate validates if this requirement is fullfilled for the provided global flags.
+// It should return either nil, or an error of type exit.Error.
+//
+// Validate does not take into account AllowsOption, see ValidateAllowedOptions.
+func (r Requirements) Validate(arguments goprogram.Arguments[struct{}]) error {
+	return nil
+}