This adds an optional feature LEJP_FLAG_FEAT_OBJECT_INDEXES that changes
lejp to treat { } items as indexable in ctx->i[] / ctx->ipos, since they
also can take commas.
This may break existing uses so it requires the default-off feature flag to
enable it. The flags context field is zeroed by lejp_construct(), so any
flags should be set on ctx->flags after alling that.
There's also a flag LEJP_FLAG_LATEST available as an alias to enable any
desirable but not-backwards-compatible behaviour, including this.
Add the info to the README and adapt the unit test to do it both with and
without the FEAT_OBJECT_INDEXES flag.