refactor the code names Follow new format

rename all files in the specific format
problem difficulty first 

> easy , mid , hard

then a ` - `

then problem number

then again a ` - `

then the problem name as it is on leetcode but replace the space with ` _ `

so for example for problem like ` 3442.kill the murderer ` which is hard

it will be ` hard-3442-kill_the_murderer.(whichever extension it had previously `
