Change the linter match to a regex in the file name when "domatch=true"

[taconi]

Previously, the match was done with a lua pattern in the file type.
The change is for the pattern to be a regular expression that will match the full file path.

This way, linters can be added to files that do not have a type defined in the micro editor and allow the user to use regular expressions.

Example of a linter for files ending with .rb:

function init()
  linter.makeLinter(
    'rubocop',                      -- name
    '\\.rb$',  -- filetype
    'rubocop',                      -- cmd
    { '--format', 'emacs', '%f' },  -- args
    '%f:%l:%c: C: %m',              -- errorformat
    {},                             -- os
    false,                          -- whitelist
    true       -- domatch: filetype will be a regex pattern that will match the file path.
  )
end

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closes #3156