Modify a number of examples in vlang.io/docs to run correctly in Playground

[Tom Paoletti]

• 2 examples at https://vlang.io/docs#match

  undefined: `number`
    8| }

unexpected token: `enum`
    1| fn main() {

• 1 example at https://vlang.io/docs#structs

unexpected token: `}`
   11| println(p.x) // Struct fields are accessed using a dot

• 1 example at https://vlang.io/docs#methods

unexpected token: `struct`
    1| fn main() {

• 1 example at https://vlang.io/docs#recvs

unexpected token: `struct`
    1| fn main() {

• 2 examples at https://vlang.io/docs#consts

unexpected token: `}`
    8| println(world)

unexpected token: `}`
   21| println(blue)

• 1 example in https://vlang.io/docs#interfaces

unexpected token: `struct`
    1| fn main() {

• 1 example in https://vlang.io/docs#enums

unexpected token: `}`
    9| println(color) // "1"  TODO: print "green"? 

• 1 example in https://vlang.io/docs#json

undefined: `json`, use `import json`
   14| 

[Tom Paoletti]

@medvednikov Opened PR to take care of these. Thanks !