feat: added custom styling for preprocessor macros

Styles such as bold and italic can be defined for preprocessor macros.
Updated the default syntax group mapping, boilerplate config, and docs
to reflect these changes. Note: make docgen was not working for me so I
manually updated the documentation myself. Please advise if changes are
needed.

doc/nightfox.txt

@@ -266,6 +266,7 @@ options.styles {table}                 `styles` is a table that contains a list
 - keywords
 - numbers
 - operators
+- preprocs
 - strings
 - types
 - variables

lua/nightfox/config.lua

@@ -27,6 +27,7 @@ local defaults = {
     keywords = "NONE",
     numbers = "NONE",
     operators = "NONE",
+    preprocs = "NONE",
     strings = "NONE",
     types = "NONE",
     variables = "NONE",

lua/nightfox/group/syntax.lua

@@ -27,7 +27,7 @@ function M.get(spec, config)
     Keyword        = { fg = syn.keyword, style = stl.keywords }, -- any other keyword
     Exception      = { link = "Keyword" }, -- try, catch, throw
 
-    PreProc        = { fg = syn.preproc }, -- (preferred) generic Preprocessor
+    PreProc        = { fg = syn.preproc, style = stl.preprocs }, -- (preferred) generic Preprocessor
     Include        = { link = "PreProc" }, -- preprocessor #include
     Define         = { link = "PreProc" }, -- preprocessor #define
     Macro          = { link = "PreProc" }, -- same as Define

usage.md

@@ -208,6 +208,7 @@ combination of |highlight-args|. The list of syntax components are:
 - keywords
 - numbers
 - operators
+- preprocs
 - strings
 - types
 - variables