tools: add v ast file.v (#10236)

cmd/tools/vast/test/.gitignore

@@ -0,0 +1 @@
+demo.json

cmd/tools/vast/test/demo.v

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+// usage test: v ast path_to_v/cmd/tools/vast/test/demo.v
+// will generate demo.json
+
+// comment for module
+module main
+
+// import module
+import os
+import math
+import time { Time, now }
+
+// const decl
+const (
+	a = 1
+	b = 3
+	c = 'c'
+)
+
+// struct decl
+struct Point {
+	x int
+mut:
+	y int
+pub:
+	z int
+pub mut:
+	name string
+}
+
+// method of Point
+pub fn (p Point) get_x() int {
+	return p.x
+}
+
+// embed struct
+struct MyPoint {
+	Point
+	title string
+}
+
+// enum type
+enum Color {
+	red
+	green
+	blue
+}
+
+// type alias
+type Myint = int
+
+// sum type
+type MySumType = bool | int | string
+
+// function type
+type Myfn = fn (int) int
+
+// interface type
+interface Myinterfacer {
+	add(int, int) int
+	sub(int, int) int
+}
+
+// main funciton
+fn main() {
+	add(1, 3)
+	println(add(1, 2))
+	println('ok') // comment println
+	arr := [1, 3, 5, 7]
+	for a in arr {
+		println(a)
+		add(1, 3)
+	}
+	color := Color.red
+	println(color)
+	println(os.args)
+	m := math.max(1, 3)
+	println(m)
+	println(now())
+	t := Time{}
+	println(t)
+	p := Point{
+		x: 1
+		y: 2
+		z: 3
+	}
+	println(p)
+	my_point := MyPoint{
+		// x: 1
+		// y: 3
+		// z: 5
+	}
+	println(my_point.get_x())
+}
+
+// normal function
+fn add(x int, y int) int {
+	return x + y
+}
+
+// generic function
+fn g_fn<T>(p T) T {
+	return p
+}
+
+// generic struct
+struct GenericStruct<T> {
+	point Point
+mut:
+	model T
+}