fix: parse interpolated vocal shorthand

[kyui-azusa]

Summary

• recognize an unescaped runtime interpolation as vocal shorthand when no explicit argument value is present
• pass shorthand such as -{voiceFile} through the vocal asset path resolver
• add a parser regression test for the interpolated shorthand form

Validation

• yarn parser:test --run (30 tests passed)
• yarn parser:build
• yarn webgal:build

Closes #888