strings: rename Builder.write_b() to Builder.write_byte(), add deprecation (#13313)

vlib/strings/builder.c.v

@@ -49,10 +49,17 @@ pub fn (mut b Builder) write_runes(runes []rune) {
 }
 
 // write_b appends a single `data` byte to the accumulated buffer
+[deprecated: 'Use write_byte() instead']
+[deprecated_after: '2022-02-11']
 pub fn (mut b Builder) write_b(data byte) {
 	b << data
 }
 
+// write_byte appends a single `data` byte to the accumulated buffer
+pub fn (mut b Builder) write_byte(data byte) {
+	b << data
+}
+
 // write implements the Writer interface
 pub fn (mut b Builder) write(data []byte) ?int {
 	if data.len == 0 {