syntax: Update syntax elements for jsonnet v0.19.1 (#33)

Update the standard library names and add the importbin statement to update the syntax highlighting to jsonnet v0.19.1.

The standard library names have been automatically generated with the following command:

PROG='std.lines(["syn match Constant \"std.%s\"" % fn for fn in std.objectFieldsAll(std) if fn[0] >= "a" && fn[0] <= "z"])''
go run github.com/google/go-jsonnet/cmd/jsonnet@v0.19.1 --string - <<<"${PROG}"

This should make it easier to keep up-to-date with additions to the standard library, as it was missing some rather old additions. Seems to have been rather adhoc.

As part of this automation, the builtins and std.jsonnet have been combined as there is no way to determine which names are builtin and which come from std.jsonnet. Manually maintaining the distinction is pointless busywork, and that fell behind anyway as a bunch of std.jsonnet functions have been migrated to builtins for performance reasons.

The automation script lists the names in the std object, ignoring all names that do not start with a lower-case letter. This removes some internal names that should not be added to the syntax highlighting.

The importbin statement was manually added.