Addition of logger #6

[AntoineGS]

Addition of a new unit (logger.lua) to centralize the logging and the notify processes, including new configurations to trace and log messages from the LSP.

config:

  logger = {
    log_to_file = false,
    file = vim.fn.stdpath("log") .. "/copilot-lua.log",
    file_log_level = vim.log.levels.WARN,
    print_log = true,
    print_log_level = vim.log.levels.WARN,
    trace_lsp = "off", -- "off" | "messages" | "verbose"
    trace_lsp_progress = false,
  },

From the Readme:

logger

When log_to_file is true, logs will be written to the file for anything of file_log_level or higher.
When print_log is true, logs will be printed to NeoVim (using notify) for anything of print_log_level or higher.
File logging is done asynchronously to minimize performance impacts, however there is still some overhead.

Log levels used are the ones defined in vim.log:

vim.log = {
  levels = {
    TRACE = 0,
    DEBUG = 1,
    INFO = 2,
    WARN = 3,
    ERROR = 4,
    OFF = 5,
  },
}

trace_lsp can either be off, messages which will output the LSP messages, or verbose which adds additonal information to the message.
When trace_lsp_progress is true, LSP progress messages will also be logged.

Careful turning on all logging features as the log files may get very large over time, and are not pruned by the application.

Fixes #356