Breaking change on `grep` and `grep2`

[liuchengxu]

I observe that a lot of people still use grep over grep2, due to historical reasons, they are likely unaware that grep2 is much faster and responsive in most cases due to the cache strategy and people are indeed generally more comfortable with typing grep instead of grep2. In order to encourage people to switch to grep2, I'm planning to rename the provider grep2 to plain grep and rename the legacy grep to live_grep, which also indicates the subtle difference between these two providers: live_grep is just like invoking the ripgrep executable in the command line which uses the ripgrep's matcher, but grep uses the fuzzy matcher in vim-clap.

• :Clap grep => :Clap live_grep
• :Clap grep2 => :Clap grep