runtime(json): Add json formating plugin (Issue #11426) (#11506)

related: #11426

Signed-off-by: Maxim Kim <habamax@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/doc/filetype.txt

@@ -610,6 +610,18 @@ The mapping can be disabled with: >
 	let g:no_gprof_maps = 1
 
 
+JSON-FORMAT						*ft-json-plugin*
+
+JSON filetype can be extended to use 'formatexpr' and "json.FormatExpr()"
+function for json formatting (using |gq|).
+
+Add following lines to $HOME/.vim/ftplugin/json.vim: >
+
+	vim9script
+	import autoload 'dist/json.vim'
+	setl formatexpr=json.FormatExpr()
+
+
 MAIL							*ft-mail-plugin*
 
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