add tests for the parser, smart dates, and recurrence

There is no *_test.go anywhere in the repo.

ParseTask, compileSmartDate, and generateRecurringTask are effect-free and full of edge cases — priority/modifier handling, +project/@context tokens, due:/rec:, +Nd/+Nw/+Nm/+Ny, month and year rollover, leap years — with zero coverage.

Table-driven tests here would also give us somewhere to reproduce the bulk complete/delete ID bug before fixing it.