[feature] support nested configuration files, and setting ALL configuration variables by CLI and env (#4109)

This updates our configuration code generator to now also include map marshal and unmarshalers. So we now have much more control over how things get read from pflags, and stored / read from viper configuration. This allows us to set ALL configuration variables by CLI and environment now, AND support nested configuration files. e.g.

```yaml
advanced:
    scraper-deterrence = true

http-client:
    allow-ips = ["127.0.0.1"]
```

is the same as

```yaml
advanced-scraper-deterrence = true

http-client-allow-ips = ["127.0.0.1"]
```

This also starts cleaning up of our jumbled Configuration{} type by moving the advanced configuration options into their own nested structs, also as a way to show what it's capable of. It's worth noting however that nesting only works if the Go types are nested too (as this is how we hint to our code generator to generate the necessary flattening code :p).

closes #3195

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4109
Co-authored-by: kim <grufwub@gmail.com>
Co-committed-by: kim <grufwub@gmail.com>

vendor/codeberg.org/gruf/go-split/join_util.go

@@ -0,0 +1,71 @@
+package split
+
+import (
+	"strconv"
+	"strings"
+)
+
+// singleTermLine: beyond a certain length of string, all of the
+// extra checks to handle quoting/not-quoting add a significant
+// amount of extra processing time. Quoting in this manner only really
+// effects readability on a single line, so a max string length that
+// encompasses the maximum number of columns on *most* terminals was
+// selected. This was chosen using the metric that 1080p is one of the
+// most common display resolutions, and that a relatively small font size
+// of 7 requires ~ 223 columns. So 256 should be >= $COLUMNS (fullscreen)
+// in 99% of usecases (these figures all pulled out of my ass).
+const singleTermLine = 256
+
+// appendQuote will append 'str' to 'buf', double quoting and escaping if needed.
+func appendQuote(buf []byte, str string) []byte {
+	switch {
+	case len(str) > singleTermLine || !strconv.CanBackquote(str):
+		// Append quoted and escaped string
+		return strconv.AppendQuote(buf, str)
+
+	case (strings.IndexByte(str, '"') != -1):
+		// Double quote and escape string
+		buf = append(buf, '"')
+		buf = appendEscape(buf, str)
+		buf = append(buf, '"')
+		return buf
+
+	case (strings.IndexByte(str, ',') != -1):
+		// Double quote this string as-is
+		buf = append(buf, '"')
+		buf = append(buf, str...)
+		buf = append(buf, '"')
+		return buf
+
+	default:
+		// Append string as-is
+		return append(buf, str...)
+	}
+}
+
+// appendEscape will append 'str' to 'buf' and escape any double quotes.
+func appendEscape(buf []byte, str string) []byte {
+	var delim bool
+	for i := range str {
+		switch {
+		case str[i] == '\\' && !delim:
+			// Set delim flag
+			delim = true
+
+		case str[i] == '"' && !delim:
+			// Append escaped double quote
+			buf = append(buf, `\"`...)
+
+		case delim:
+			// Append skipped slash
+			buf = append(buf, `\`...)
+			delim = false
+			fallthrough
+
+		default:
+			// Append char as-is
+			buf = append(buf, str[i])
+		}
+	}
+	return buf
+}