Support flag aliases or "flag-based defaults" for CLI arguments

[stychu]

Currently, when defining flags in mise (specifically within task configurations or custom scripts), the default key typically accepts a literal value.

I would like to be able to define the default state of a flag using the flag syntax itself, or provide a way to map a flag to a set of predefined "default" arguments. This allows for more expressive configurations where a simple flag can represent a more complex internal state.

Proposed Syntax
I'd like to be able to define defaults like this:

[tasks.example]
# Current behavior (literal)
flag "--force" = { default = true }

# Proposed behavior (flag-based default)
flag "-f --force" = { default = "--with-force" }

Context / Use Case
In many CLI tools, a flag isn't just a boolean true/false, but a shorthand for a specific mode of operation. By allowing the default to be defined as a string of flags (or an alias), we can:

Standardize Tooling: Map mise flags directly to the underlying tool's more verbose flags.

Simplify Task Definitions: Users won't have to manually parse strings if the default can be passed directly as a pre-configured flag state.

Examples
If I have a task called deploy:

Defining flag "--mode" = { default = "--production" }

Running mise run deploy would effectively execute the command with the --production context.

Running mise run deploy --mode --staging would override that default.