examples: add web_crawler and get_weather (#10084)

examples/get_weather/README.md

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+# get_weather
+get_weather is a very simple web crawler.
+Its goal is to get a weather forecast from caiyunapp.com.  
+
+# Compile and Run
+
+Use this to generate an executable and then launch the web crawler.
+```bash
+v get_weather.v
+./get_weather
+```
+
+As a convenience, you can also compile and launch the web crawler directly.
+```bash
+v run get_weather.v
+```
+
+In this project we use http.fetch() to get a http.Response, with a
+custom user-agent and then we use json.decode() to decode the json 
+response to struct.
+We also use a `[skip]` attribute to skip certain fields in the response,
+that we don't need and use a `[json: result]` attribute to specify that
+our struct field is named differently from the incoming json response.

examples/get_weather/get_weather.v

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+import json
+import rand
+import net.http
+
+struct Weather {
+	status      string [skip] // drop this field
+	api_version string [skip]
+	api_status  string [skip]
+	lang        string [skip]
+	unit        string [skip]
+	tzshift     int    [skip]
+	timezone    string [skip]
+	server_time u32    [skip]
+	location    []f32  [skip]
+	result      Result //[json: result] if the field name is different in JSON, it can be specified
+}
+
+struct Result {
+	realtime          Realtime [skip]
+	minutely          Minutely [skip]
+	hourly            Hourly   [skip]
+	daily             Daily    [skip]
+	primary           int      [skip]
+	forecast_keypoint string
+}
+
+struct Realtime {}
+
+struct Minutely {}
+
+struct Hourly {}
+
+struct Daily {}
+
+fn main() {
+	config := http.FetchConfig{
+		user_agent: 'Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:88.0) Gecko/20100101 Firefox/88.0'
+	}
+
+	rnd := rand.f32()
+	url := 'https://api.caiyunapp.com/v2.5/96Ly7wgKGq6FhllM/116.391912,40.010711/weather.jsonp?hourlysteps=120&random=$rnd'
+	// println(url)
+
+	resp := http.fetch(url, config) or {
+		println('failed to fetch data from the server')
+		return
+	}
+
+	weather := json.decode(Weather, resp.text) or {
+		println('failed to decode weather json')
+		return
+	}
+
+	println('未来两小时天气:\n${weather.result.forecast_keypoint}.')
+}