Add ability to configure the file modification indicator

[cmattinson]

Relates to the second request in #7481.

Previously the file modification indicator was documented under [editor.statusline], but the way I have it currently, the configuration will change the file modification indicator on both the buffer line AND the status line. I've moved it to [editor] as it's more of a general editor change the way I have it set up. Wondering if there are any opinions on whether:

A - The configurable indicator should control both the buffer and status line indicator (current)
B - The configurable indicator should only be for the buffer line and the status line default [+] should remain
C - They should be independently configurable

I've also made the assumption that someone may want the entire indicator configurable, rather than just the character(s) inside the [].

[sharpenedblade]

I think the indicator being independently configurable is the best way to go about this. The indicators could be different because the bufferline needs to fit more buffer names but the statusline only needs to fit one (example: [m] in the bufferline and [modified] in the statusline).