Automatically pad spaces to align icons of the same level

[Bekaboo]

Description

First of all thanks for making such an awesome plugin! From time to time I find myself prefer using pure ASCII text instead of nerd icons, but using the first character of kind names (as suggested in README.md) is not informative, so I use the abbreviations of kind names instead for the symbol icons. However, the abbreviations are variable-length, which makes the variable names misaligned, as shown in the screenshot below.

Could we pad the symbol icons in the outline window so that symbols on the same level are always aligned?

[hedyhli]

Hey, although this is a reasonable request, it's something that can be achieved through the config by padding spaces yourself:

symbols = {
  icons = {
    Function  = { icon = 'Func', hl = 'Function' },
    Variable  = { icon = 'Var ', hl = 'Constant' },
    Constant  = { icon = 'Con ', hl = 'Constant' },
    String    = { icon = 'Str ', hl = 'String' },
    Number    = { icon = 'Num ', hl = 'Number' },
    Boolean   = { icon = 'Bool', hl = 'Boolean' },
    ...
  }
}

In this example, we want all icons to have a width of 4 characters.

In fact, before I gave in and set up nerdicons in my terminal, I have used 4-character icons for both symbols and the Vim mode indicator (norm/ins /vis /VL ) and would pad them myself (I'm also not aware of a statusline plugin that automatically pads the mode indicator section, correct me if I'm wrong).

Sometimes I would want to update those text icons, such as switching to 2 characters instead. By aligning those strings we can easily use the Visual Block mode to bulk-edit the icon strings to insert and remove spaces :)

I'm happy to update the default config (in the readme) or accept PRs to align the keys of the symbols.icons field like in the example above.