examples: add web_crawler and get_weather (#10084)

examples/file_list.v

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+import os
+
+fn main() {
+	files := os.ls('.') or {
+		println(err)
+		return
+	}
+	mut f := os.create('file_list.txt') or {
+		println(err)
+		return
+	}
+	for file in files {
+		if os.is_file(file) {
+			f.write_string(file + '\r\n') or { println(err) }
+		}
+	}
+	f.close()
+}

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}.')
+}

examples/web_crawler/README.md

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+# web_crawler
+web_crawler is a very simple web crawler.  
+This web crawler fetches news from tuicool.com,
+(a chinese site similar to hacker-news.firebaseio.com).  
+
+# Compile and Run
+
+Use this to generate an executable, and then launch the web crawler:
+```bash
+v web_crawler.v
+./web_crawler
+```
+
+And this to compile and launch the web crawler directly:
+```bash
+v run web_crawler.v
+```
+
+This project shows how to use http.fetch() to get http.Response, 
+and then html.parse() to parse the returned html.
+
+It's easy, isn't it?  

examples/web_crawler/web_crawler.v

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+import net.http
+import net.html
+
+fn main() {
+	/*
+	user_agent = 'v.http'
+    resp := http.get('https://tuicool.com') or {
+        println('failed to fetch data from the server')
+        return
+    }
+	*/
+	// http.fetch() sends an HTTP request to the URL with the given method and configurations.
+	config := http.FetchConfig{
+		user_agent: 'Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:88.0) Gecko/20100101 Firefox/88.0'
+	}
+	resp := http.fetch('https://tuicool.com', config) or {
+		println('failed to fetch data from the server')
+		return
+	}
+	// html.parse() parses and returns the DOM from the given text.
+	mut doc := html.parse(resp.text)
+	// html.DocumentObjectModel.get_tag_by_attribute_value() retrieves all the tags in the document that has the given attribute name and value.
+	tags := doc.get_tag_by_attribute_value('class', 'list_article_item')
+	for tag in tags {
+		href := tag.children[0].attributes['href'] or { panic('key not found') }
+		title := tag.children[0].attributes['title'] or { panic('key not found') }
+		println('href: $href')
+		println('title: $title')
+		println('')
+	}
+}