FormatJson: Make it possible to change the indent string

This is to allow consistency with MediaWiki PHP and JS files (e.g. when
generating JSON i18n files), not because tabs are "better" than spaces for
indenting code (both have advantages and disadvantages).

Because PHP's json_encode() function hardcodes the indent string, using tabs
has a performance cost (in post-processing the output) and is less suitable
for web output; thus the API and ResourceLoader debug mode will continue to
use four spaces. Adjusting the maintenance scripts and JSON files is left to
separate change sets.

Bug: 63444
Change-Id: Ic915c50b0acd2e236940b70d5dd48ea87954c9d5