Support for some format variants

[noritada]

This PR adds support for some format variants.

Some of the systems that generate data in the WNI RU format generate data in variant formats,
which are slightly different from the format specified in the specifications referenced in the development of this library.
This PR adds support for following 3 cases:

• Allow a trailing comma in the schema description
• Allow an empty field name in the schema description when it is the only field in the schema
• Allow use of <N>STR instead of <N>NSTR in the schema description

Each of these cases can be optionally enabled using flags.

All of these options are enabled in the CLI and the web demo app.

[noritada]

Self-reviewed. Passing CI.