schema: allow singular precondition on task

[philpennock]

The docs explicitly show using a singular "precondition" on a Task at
https://taskfile.dev/reference/schema/#precondition, but the schema only
allowed the plural "preconditions" as a list.

This updates the schema to match the documented easy path.

---

Examples

Given a Task of:

  format:
    desc: Format the config files for consistency
    precondition: test -f {{.POLICY_FILE}}
    cmds:
      - hujsonfmt -w {{.POLICY_FILE}}

then running task format runs just fine. But without this change, my text
editor displays:

E     precondition: test -f {{.POLICY_FILE}}     ■ Property precondition is not allowed.

Switching to a local schema file and making the change in this PR, the warning
disappears and LSP/yamlls in neovim agrees with Task about what's valid.

[vmaerten]

Hello,
It seems the documentation is not up to date. precondition is not allowed. Only preconditions is.

Your example is inccorect. It does not apply precondition.

cf :

[philpennock]

Oh drat. I didn't realize that task ignored fields it didn't understand and I didn't adequately test. Okay. This is bogus, sorry.