powershell completion: disable file path autocomplete / add flags

[trim21]

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Didn't know there is a command task --list-all --json/--silent for all available tasks when I submit last PR.

Previous, if your pwd have a dir or file named gen or gen..., and you have a task named gen,
when you type task gen<tab>, powershell will fallback to filepath complete, and your input will become task .\gen

after this pr, task gen<tab> are still task gen

File path auto completion still works with .\<tab> or ./<tab>

[trim21]

I try to add short opt but looks like old powershell (5.1, win10 default version) doesn't work will with that, So I'll just add long flags.

[trim21]

adding description feature, convert to draft

[trim21]

description feature make it much slower to finish completion ( caused by parsing json ), so I removed it.

[andreynering]

Thanks!