Add gha shell for GitHub Actions

GHA has both simple and multi-line env var modes, I decided to always use the
multiline mode for ease of implementation.

internal/cmd/shell.go

@@ -48,6 +48,8 @@ func DetectShell(target string) Shell {
 		return Elvish
 	case "fish":
 		return Fish
+	case "gha":
+		return GitHubActions
 	case "gzenv":
 		return GzEnv
 	case "json":

internal/cmd/shell_gha.go

@@ -0,0 +1,53 @@
+package cmd
+
+import (
+	"fmt"
+	"strings"
+)
+
+type gha struct{}
+
+// GitHubActions shell instance
+var GitHubActions Shell = gha{}
+
+func (sh gha) Hook() (string, error) {
+	return "", fmt.Errorf("Hook not implemented for GitHub Actions shell")
+}
+
+func (sh gha) Export(e ShellExport) string {
+	var b strings.Builder
+	for key, value := range e {
+		if value == nil {
+			sh.unset(&b, key)
+		} else {
+			sh.export(&b, key, *value)
+		}
+	}
+	return b.String()
+}
+
+const ghaDelim = "DIRENV_GITHUB_ACTIONS_EOV\n"
+
+func (sh gha) Dump(env Env) string {
+	var b strings.Builder
+
+	for key, value := range env {
+		sh.export(&b, key, value)
+	}
+	return b.String()
+}
+
+func (sh gha) export(b *strings.Builder, key, value string) {
+	b.WriteString(key)
+	b.WriteString("<<")
+	b.WriteString(ghaDelim)
+	b.WriteString(value)
+	if value != "" && !strings.HasSuffix(value, "\n") {
+		b.WriteByte('\n')
+	}
+	b.WriteString(ghaDelim)
+}
+
+func (sh gha) unset(b *strings.Builder, key string) {
+	sh.export(b, key, "")
+}