[enh] Add command line engines: git grep, find, etc. (#2128)

A new "base" engine called command is introduced. It is the foundation for all command line engines for now.
You can use this engine to create your own command line engine.

Add some engines (commented out to make sure no one enables anything accidentally):
* git grep: This engine lets you grep in the searx repo.
* locate: If locate is installed and initialized, you can search on the FS.
* find: You can find files with a specific name from where you started searx.
* pattern search in files: This engine utilizes the command fgrep.
* regex search in files: This engine runs `grep` to find a file based on its contents.

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