Fix custom instruments getting empty name and ID

[Bentroen]

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Closes #8

Information

As per issue #8, during the parsing of an NBS file, custom instruments are generated that contain empty meta.name (empty string) and id (NaN) attributes. This results from the name string being read from the file, but not actually stored in the name attribute, which receives the default value "" from the defaultInstrumentOptions object.

Additionally, as the first parameter to the Instrument object constructor is passed the return value of Number.parseInt called on the parsed string, which always results in NaN. As a result, this value is assigned to all custom instruments' id attributes.

This pull request fixes the issue by calculating the custom instrument's ID from the firstCustomIndex property and the index of the current instrument being read, and assigning the parsed name string to the meta.name property.

[encode42]

Thanks for the research! This happened during a minor refactor for v4. I don't know how this was missed. Clearly I need to write more tests, but that'll hopefully be dealt with soon when I start using the project more myself.