feat: add environ format

Adds a `direnv export environ` command that outputs the diff in
`<key>(=<value)?\0` format. It's a simple format that is quite close to
/proc/<pid>/environ on Linux. Each line is NUL-terminated. If the `=` is
not present on a line, consider that the <key> should be unset.

The idea was to find a format that could eventually replace the evals in
the hooks in the various shells.

internal/cmd/shell.go

@@ -46,6 +46,8 @@ func DetectShell(target string) Shell {
 		return Bash
 	case "elvish":
 		return Elvish
+	case "environ":
+		return Environ
 	case "fish":
 		return Fish
 	case "gha":

internal/cmd/shell_environ.go

@@ -0,0 +1,51 @@
+package cmd
+
+import (
+	"errors"
+)
+
+// environShell is not a real shell
+type environShell struct{}
+
+// Environ is a format exporter and not a shell. It implements a similar
+// format as /proc/<pid>/environ in Linux.
+var Environ Shell = environShell{}
+
+func (sh environShell) Hook() (string, error) {
+	return "", errors.New("this feature is not supported")
+}
+
+// Exports emits a string composed of <key>(=<value>), separated by \0.
+//
+// It's the same as the Dump format with one additional case: if there is no =
+// in the line, the environment variable should be unset.
+func (sh environShell) Export(e ShellExport) (out string) {
+	for key, value := range e {
+		if value == nil {
+			out += sh.unset(key)
+		} else {
+			out += sh.export(key, *value)
+		}
+	}
+	return out
+}
+
+// Dump emits a string composed of <key>=<value, separated by \0. That
+// format is the same as you would find in /prod/<pid>/environ and works with
+// values that include special characters like \n.
+func (sh environShell) Dump(env Env) (out string) {
+	for key, value := range env {
+		out += sh.export(key, value)
+	}
+	return out
+}
+
+// <key>=<value> , terminated by \0
+func (sh environShell) export(key, value string) string {
+	return key + "=" + value + string([]byte{0})
+}
+
+// <key> , terminated by \0
+func (sh environShell) unset(key string) string {
+	return key + string([]byte{0})
+}