encoding/json: use append for HTMLEscape

encoding/json: use append for HTMLEscape

Use append for HTMLEscape similar to Indent and Compact.
Move it to indent.go alongside Compact, as it shares similar logic.
In a future CL, we will modify appendCompact to be written in terms
of appendHTMLEscape, but we need to first move the JSON decoder logic
out of the main loop of appendCompact.

Change-Id: I131c64cd53d5d2b4ca798b37349aeefe17b418c7
Reviewed-on: https://go-review.googlesource.com/c/go/+/471198
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>