Support rest_parameter, optional_parameter, default value of parameter

[WindSoilder]

This pr adds support for defining custom commands with custom completion, option parameter and default values, here are examples:

def foo-optional [x: int, y?: int] { [$x, $y] }
def foo-rest [x: int, ...y] { [$x, $y] }
def completion [] { [1, 2, 3] }
def foo-with-completion [--bar: int@completion, baz: int@completion] { [$bar, $baz] }

To achieve this, I stored more infomation into some variants of AstNode:

• AstNode::FlagParam: custom_completion field
• AstNode::PosParam: custom_completion, is_option fields
• AstNode::RestParam: a new variant to store rest parameter information.

It's also one-to-one mapping to grammar.md.

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