Remove company dependency

[gmlarumbe]

company was previously required to handle the limitation of gathering completions from multiple backends/functions with completion-at-point:

• The default for comint is to set completion-at-point-functions to comint-dynamic-complete-functions
• In turn, comint-dynamic-complete-functions has the default value '(comint-c-a-p-replace-by-expanded-history comint-filename-completion). These functions work as if they were completion-at-point-functions themselves, returning completions if found and returning nil and checking next function if not. The recommended approach is to add a new capf function to comint-dynamic-complete-functions.
• If completion for both, files and commands, is set with two different functions, results found by one could mask the results found by the other. For example:
  • Consider current shell $PWD is /home/user
  • Type "r"
  • Try completions:
    • First function is called: comint-filename-completion
      • Finds a directory named "repos" and returns it as a completion candidate
    • Second function fpga-*-capf is not called since previous one was already successful
      • Commands like "run" or "remove" will be missed
    • In this case commands are masked by files. Placing commands function before the files one in the hook would also have a masking effect.

The PR implements a single capf function that handles both files and commands.

It also adds a customizable variable: fpga-utils-shell-file-completion, if file completion inside capf is not desired. This could be the case if using company-files backend with the following snippet:

(setq-local comint-dynamic-complete-functions '(fpga-xilinx-vivado-shell-capf))
(setq-local company-backends '(company-files company-capf))