Enable JSON1 for bundled libsqlite

[KristofMorva]

As JSON spreads very fast as a configuration-file and general communication format, in the past few years almost every SQL language has developed their own JSON support: MySQL, PostgreSQL, MariaDB, and even SQLite. However, while these can be used in all other drivers out-of-the-box, SQLite is a weak exception regarding this: JSON is supplied with an external plug-in. It'd be great if this extremely useful feature would be part of the core bundle. The code base is quite small compared to the FTS plug-ins, so it would not cause unnecessary overhead. It also does not introduce new types or operators - only a few JSON_* functions.

@weltling @remicollet do you think it's viable?

[krakjoe]

ping @remicollet @weltling ATT

[cmb69]

That would affect the bundled sqlite only, and I think in the Linux world usually ext/sqlite3 is build against a sytem lib. OTOH, since we're enabling the FTS plugins for the bundled lib, enabling Json1 would also be okay.

[weltling]

Looks good, merged as ed0a58d into master as there was no demand on this before. Tested with bundled lib, when a linked one has it - will work too. The doc say's it's available as of 3.9.0. Lower branches might be still safe enabling this, so can be backported.

Thanks.