implement live-reloading dev server

Squashed commit of the following:

commit 02f077432202af4d633eb2cad81dfdaa6921317f
Author: Alan Pearce <alan@alanpearce.eu>
Date:   Sat Apr 27 21:09:14 2024 +0200

    builder: only remove output directory if set and in dev mode

commit 47001e01c55fa6e74aafeda04ebc3e4e7c47eba0
Author: Alan Pearce <alan@alanpearce.eu>
Date:   Sat Apr 27 21:03:37 2024 +0200

    implement live reload on dev server

commit 411ec969f61e4b73439f1c54ea29f75135ecc618
Author: Alan Pearce <alan@alanpearce.eu>
Date:   Sat Apr 27 20:59:26 2024 +0200

    server: implement graceful shutdown

commit 5400132eb6eb1b638e0b3fd4265f51611c92d473
Author: Alan Pearce <alan@alanpearce.eu>
Date:   Sat Apr 27 20:41:07 2024 +0200

    add some debug logs

commit 3c9b678197c044603950232d222f501ef74d7873
Author: Alan Pearce <alan@alanpearce.eu>
Date:   Sat Apr 27 20:39:09 2024 +0200

    prefix log output with executable name

commit 300e24c179e390e9d3f5aeab4471c97f17f1fa64
Author: Alan Pearce <alan@alanpearce.eu>
Date:   Sat Apr 27 20:29:42 2024 +0200

    don't panic inside internal packages, return error instead

commit fe2715d330402ad67fe866471bed89c7238ad2ec
Author: Alan Pearce <alan@alanpearce.eu>
Date:   Fri Apr 26 01:18:29 2024 +0200

    config: use a table to configure CSP headers

commit d012553aaf78a436fa8871830b5d720a9e292d4b
Author: Alan Pearce <alan@alanpearce.eu>
Date:   Thu Apr 25 17:13:39 2024 +0200

    dev: create basic dev server to build and serve from a temporary directory

commit a1d11d3e69650d9b43ca1b1d7b7ccc05a808d5c1
Author: Alan Pearce <alan@alanpearce.eu>
Date:   Thu Apr 25 13:02:22 2024 +0200

    remove unused redirect_other_hostnames config option

commit fd67b19b5c7f76f0c3579e8a05ef20a618e90be7
Author: Alan Pearce <alan@alanpearce.eu>
Date:   Thu Apr 25 12:58:53 2024 +0200

    server: make port a string, which is what go uses

commit c798e8e736c0649008cade337158399470a9099b
Author: Alan Pearce <alan@alanpearce.eu>
Date:   Thu Apr 25 12:58:33 2024 +0200

    config: remove unused port variable

commit f94882b9001f3b0855e26b26b4a84b96e3deb22b
Author: Alan Pearce <alan@alanpearce.eu>
Date:   Thu Apr 25 12:49:10 2024 +0200

    re-organise module layout

1 files changed, 77 insertions, 0 deletions

diff --git a/internal/server/filemap.go b/internal/server/filemap.go
new file mode 100644
index 0000000..466db49
--- /dev/null
+++ b/internal/server/filemap.go
@@ -0,0 +1,77 @@
+package server
+
+import (
+	"fmt"
+	"hash/fnv"
+	"io"
+	"io/fs"
+	"log"
+	"log/slog"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/pkg/errors"
+)
+
+type File struct {
+	filename string
+	etag     string
+}
+
+var files = map[string]File{}
+
+func hashFile(filename string) (string, error) {
+	f, err := os.Open(filename)
+	if err != nil {
+		return "", err
+	}
+	defer f.Close()
+	hash := fnv.New64a()
+	if _, err := io.Copy(hash, f); err != nil {
+		return "", err
+	}
+	return fmt.Sprintf(`W/"%x"`, hash.Sum(nil)), nil
+}
+
+func registerFile(urlpath string, filepath string) error {
+	if files[urlpath] != (File{}) {
+		log.Printf("registerFile called with duplicate file, urlPath: %s", urlpath)
+		return nil
+	}
+	hash, err := hashFile(filepath)
+	if err != nil {
+		return err
+	}
+	files[urlpath] = File{
+		filename: filepath,
+		etag:     hash,
+	}
+	return nil
+}
+
+func registerContentFiles(root string) error {
+	err := filepath.WalkDir(root, func(filePath string, f fs.DirEntry, err error) error {
+		if err != nil {
+			return errors.WithMessagef(err, "failed to access path %s", filePath)
+		}
+		relPath, err := filepath.Rel(root, filePath)
+		if err != nil {
+			return errors.WithMessagef(err, "failed to make path relative, path: %s", filePath)
+		}
+		urlPath, _ := strings.CutSuffix(relPath, "index.html")
+		if !f.IsDir() {
+			slog.Debug("registering file", "urlpath", "/"+urlPath)
+			return registerFile("/"+urlPath, filePath)
+		}
+		return nil
+	})
+	if err != nil {
+		return err
+	}
+	return nil
+}
+
+func GetFile(urlPath string) File {
+	return files[urlPath]
+}