feat: add option on how to get a devicon

[Futarimiti]

Within the filetype component, Lualine always gets filetype devicon by file path, and fallback to using filetype only when unsuccessful. This design leads to problems when:

1. Changing the filetype--the accompanying devicon will not be updated, if displayed Bug: filetype icon doesn't update when filetype is changed #537
2. (More practically) Editing a file with an extension shared across multiple filetypes; a notable example is .pl which is used by both Prolog and Perl. Merely checking the file path (devicons.get_icon(vim.fn.expand('%:t'))) will always result in Perl's devicon regardless actual filetype

My proposal is to allow user decide how should the devicon be determined with option get_icon_by, with valid inputs 'filename' (traditional behaviour) or 'filetype' (prioritising get_icon_by_filetype over get_icon). I'm uncertain if this change should become the default behaviour; feel free to make it so if there's no strong reason.