🎨 Introduce contextualized logging

This commit is multi-fold as it also refactors internally context and logger
as interfaces so it is easier to plug luet as a library externally.

Introduces a garbage collector (related to #227) but doesn't handle yet
parallelism.

Closes #265

pkg/api/core/logger/terminal.go

@@ -0,0 +1,43 @@
+// Copyright © 2021 Ettore Di Giacinto <mudler@mocaccino.org>
+//
+// This program is free software; you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation; either version 2 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License along
+// with this program; if not, see <http://www.gnu.org/licenses/>.
+
+package logger
+
+import (
+	"errors"
+	"os"
+
+	"github.com/mattn/go-isatty"
+	"golang.org/x/term"
+)
+
+func IsTerminal() bool {
+	return isatty.IsTerminal(os.Stdout.Fd())
+}
+
+// GetTerminalSize returns the width and the height of the active terminal.
+func GetTerminalSize() (width, height int, err error) {
+	w, h, err := term.GetSize(int(os.Stdout.Fd()))
+	if w <= 0 {
+		w = 0
+	}
+	if h <= 0 {
+		h = 0
+	}
+	if err != nil {
+		err = errors.New("size not detectable")
+	}
+	return w, h, err
+}