bug#42994: 27.1; json-serialize unable to serialize JSON values

[Jimmy Yuen Ho Wong]

According to the [JSON spec](https://www.json.org/json-en.html),
everything is a value and every value is a valid JSON, which means
literal string, numbers, and true, false and null are also valid JSON
values, but `json-serialize` is unable to serialize literals. This is
wrong and behaviorialy different from the elisp-based `json-encode`.


In GNU Emacs 27.1 (build 1, x86_64-apple-darwin19.6.0, NS appkit-1894.60 
Version 10.15.6 (Build 19G2021))
 of 2020-08-16 built on MobileCat.local
Windowing system distributor 'Apple', version 10.3.1894
System Description:  Mac OS X 10.15.6

Recent messages:
Entering debugger...
Back to top level
Entering debugger...
Back to top level
Saving file /Users/wyuenho/.emacs.d/elpa/lsp-mode-20200822.1926/lsp-eslint.el...
Wrote /Users/wyuenho/.emacs.d/elpa/lsp-mode-20200822.1926/lsp-eslint.el
Generating autoloads for 
/Users/wyuenho/.emacs.d/elpa/lsp-mode-20200822.1926/lsp-eslint.el...done
Saving file 
/Users/wyuenho/.emacs.d/elpa/lsp-mode-20200822.1926/lsp-mode-autoloads.el...
Wrote /Users/wyuenho/.emacs.d/elpa/lsp-mode-20200822.1926/lsp-mode-autoloads.el
Wrote /Users/wyuenho/.emacs.d/elpa/lsp-mode-20200822.1926/lsp-eslint.el 
(byte-compiling failed)
"true"
Configured using:
 'configure --prefix=/opt/local --without-dbus --without-gconf
 --without-libotf --without-m17n-flt --with-gpm --with-gnutls
 --with-json --with-xml2 --with-modules --infodir
 /opt/local/share/info/emacs --with-ns --with-gif --with-jpeg
 --with-lcms2 --with-png --with-tiff --with-xpm --without-harfbuzz
 --without-imagemagick --without-rsvg --without-xaw3d --with-rsvg
 'CFLAGS=-pipe -Os
 -isysroot/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk -arch
 x86_64' 'CPPFLAGS=-I/opt/local/include
 -isysroot/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk'
 'LDFLAGS=-L/opt/local/lib -Wl,-headerpad_max_install_names -Wl,-no_pie
 -Wl,-syslibroot,/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk
 -arch x86_64''

Configured features:
RSVG GLIB NOTIFY KQUEUE ACL GNUTLS LIBXML2 ZLIB TOOLKIT_SCROLL_BARS XIM
NS MODULES THREADS JSON PDUMPER LCMS2 GMP

Important settings:
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Major mode: Emacs-Lisp

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Load-path shadows:
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/Applications/MacPorts/Emacs.app/Contents/Resources/lisp/progmodes/xref
/Users/wyuenho/.emacs.d/elpa/project-0.5.1/project hides 
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/Users/wyuenho/.emacs.d/elpa/flymake-1.0.9/flymake hides 
/Applications/MacPorts/Emacs.app/Contents/Resources/lisp/progmodes/flymake
/Users/wyuenho/.emacs.d/elpa/eldoc-1.8.0/eldoc hides 
/Applications/MacPorts/Emacs.app/Contents/Resources/lisp/emacs-lisp/eldoc

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