feat: KCL Go AST definition and parser API

[Peefy]

1. Does this PR affect any open issues?(Y/N) and add issue references (e.g. "fix #123", "re #123".):

• N
• Y

• feat: KCL Go AST definition and parser API

Closes #405
Refs #404

[Peefy]

@witcxc Can you help review it and see if it is suitable for your scenario?

[coveralls]

Pull Request Test Coverage Report for Build 11888674741

Details

• 546 of 1844 (29.61%) changed or added relevant lines in 7 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.9%) to 36.303%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/parser/parser.go	7	11	63.64%
pkg/ast/type.go	0	12	0.0%
pkg/ast/stmt.go	0	72	0.0%
pkg/ast/expr.go	0	196	0.0%
pkg/ast/op.go	11	314	3.5%
pkg/ast/json.go	523	1234	42.38%

Totals
Change from base Build 11886554522:	-0.9%
Covered Lines:	5448
Relevant Lines:	15007

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💛 - Coveralls

[zong-zhe]

LGTM

[He1pa]

lgtm