Create a struct with all of Hugo's config options

Primary motivation is documentation, but it will also hopefully simplify the code.

Also,

* Lower case the default output format names; this is in line with the custom ones (map keys) and how
it's treated all the places. This avoids doing `stringds.EqualFold` everywhere.

Closes #10896
Closes #10620

common/urls/baseURL_test.go

@@ -0,0 +1,67 @@
+// Copyright 2023 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package urls
+
+import (
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+)
+
+func TestBaseURL(t *testing.T) {
+	c := qt.New(t)
+	b, err := NewBaseURLFromString("http://example.com")
+	c.Assert(err, qt.IsNil)
+	c.Assert(b.String(), qt.Equals, "http://example.com")
+
+	p, err := b.WithProtocol("webcal://")
+	c.Assert(err, qt.IsNil)
+	c.Assert(p.String(), qt.Equals, "webcal://example.com")
+
+	p, err = b.WithProtocol("webcal")
+	c.Assert(err, qt.IsNil)
+	c.Assert(p.String(), qt.Equals, "webcal://example.com")
+
+	_, err = b.WithProtocol("mailto:")
+	c.Assert(err, qt.Not(qt.IsNil))
+
+	b, err = NewBaseURLFromString("mailto:hugo@rules.com")
+	c.Assert(err, qt.IsNil)
+	c.Assert(b.String(), qt.Equals, "mailto:hugo@rules.com")
+
+	// These are pretty constructed
+	p, err = b.WithProtocol("webcal")
+	c.Assert(err, qt.IsNil)
+	c.Assert(p.String(), qt.Equals, "webcal:hugo@rules.com")
+
+	p, err = b.WithProtocol("webcal://")
+	c.Assert(err, qt.IsNil)
+	c.Assert(p.String(), qt.Equals, "webcal://hugo@rules.com")
+
+	// Test with "non-URLs". Some people will try to use these as a way to get
+	// relative URLs working etc.
+	b, err = NewBaseURLFromString("/")
+	c.Assert(err, qt.IsNil)
+	c.Assert(b.String(), qt.Equals, "/")
+
+	b, err = NewBaseURLFromString("")
+	c.Assert(err, qt.IsNil)
+	c.Assert(b.String(), qt.Equals, "")
+
+	// BaseURL with sub path
+	b, err = NewBaseURLFromString("http://example.com/sub")
+	c.Assert(err, qt.IsNil)
+	c.Assert(b.String(), qt.Equals, "http://example.com/sub")
+	c.Assert(b.HostURL(), qt.Equals, "http://example.com")
+}