feat: extract coder_secret requirements into Output (#198)

Adds a new `SecretRequirements` slice on `preview.Output`, populated by parsing `data "coder_secret" "..." { ... }` blocks in a template. Part of the User Secrets feature ([PLAT-100](https://linear.app/codercom/issue/PLAT-100)); consumed by a companion PR in `coder/coder` that renders "missing required secret" diagnostics on the create-workspace page.

feat: add `mask_input` to parameter styling options (#156)