parser: implement flag definition and calling custom command with flag

[WindSoilder]

NOTE: it's based on #74, please do not review this before it.

This pr implement flag parsing in both definition and applying, for example:

def foo-with-flag [--bar: int = 3, --baz(-b), x, y: int] { [$bar, $baz, $x, $y] }
foo-with-flag --baz

To make it easiler to do resolving and typechecking, I splitted AstNode::Param into AstNode::PosParam and AstNode::FlagParam.

It also adresses the following:

• allow custom command name contains dash(-), it's done by making call_name to return a list of berewords, which invokes identifier_allow_dash, it's one-to-one map to call_name and IDENTIFIER_ALLOW_DASH rules in grammar.md.
• allow defining default parameter for flag and parameter, it's also one-to-one map to flag_parameter and partial positional_parameter.
• allow calling command with spread operator

What's done next:

1. support rest_parameter definition
2. support option_parameter definition
3. support custom_completion definition