feat(view): indicate modified buffers (#1835)

* Outlined new options

* highlight_modified is highlight_opened_files

* prototype with autocmd

* moved modified into glyphs

* show_on_dirs and show_on_open_dirs

* icon placement before & after

* _get_filename_offset

* fixed :wq doesn't update modified indicator

* highlight_modified, signcolumn modified_placement

Refactored to make everything use HighlightedString to remove all the complex `insert_highlight` calculation.
Not tested.

* updated doc to match the reality of no multi char for glyphs.modified

* fixed git signcolumn doesn't show

* fixed highlight_modified gets replaced by highlight_opened_files

* fixed renderer.icons.show.modified = false crash

* updated doc to reflect empty icon not breaking rendering

* removed debounce_delay to implement in a later PR

* doc nit: order placement

* change modified dirs default to be consistent with git

* illegal git & modified placement changed to default

* don't assume icon exist

* nit remove comment

* Noted in doc that glyphs can't have more than 2 characters if in signcolumn

* Don't sign_define if placement isn't signcolumn

Co-authored-by: Alexander Courtis <alex@courtis.org>

lua/nvim-tree/colors.lua

@@ -40,6 +40,7 @@ local function get_hl_groups()
     SpecialFile = { gui = "bold,underline", fg = colors.yellow },
     ImageFile = { gui = "bold", fg = colors.purple },
     OpenedFile = { gui = "bold", fg = colors.green },
+    ModifiedFile = { fg = colors.green },
 
     GitDirty = { fg = colors.dark_red },
     GitDeleted = { fg = colors.dark_red },