fix(template): don't use str[1] to get 1st char

In Lua, `str[1]` on a string always returns `nil`. Use `str:find('^#')`
instead of `str[1] == '#'` to check the first byte/char.

lua/github-theme/util/template.lua

@@ -1,51 +1,52 @@
 local M = {}
 
----Walk path (one.two.three) in a table and return value
+---Walks/resolves keypath (one.two.three) against a table and returns the result.
 ---@param t table
----@param path string
+---@param keypath string
 ---@return any
-local function get_path(t, path)
-  for segment in path:gmatch('[^.]+') do
-    if type(t) == 'table' then
-      t = t[segment]
+local function get_keypath(t, keypath)
+  for segment in keypath:gmatch('[^.]+') do
+    if type(t) ~= 'table' then
+      return t
     end
+
+    t = t[segment]
   end
+
   return t
 end
 
----Parse string for configuration template
+---Parses and resolves keypath string for configuration template.
 ---@param str string
 ---@param spec Spec
 ---@return any
-local function parse_string(str, spec)
-  if str == '' then
+local function parse_keypath(str, spec)
+  if type(str) ~= 'string' or str == '' or str:find('^#') then
     return str
   end
 
-  if str[1] == '#' then
-    return str
-  end
-
-  local path = get_path(spec, str)
-  return path and path.base and path.base or path or str
+  local path = get_keypath(spec, str)
+  return path and (path.base or path) or str
 end
 
+---Resolves any and all placeholders in a template.
+---@param template table a table holding placeholders
+---@param spec table a table to resolve placeholders against
+---@return table
 function M.parse(template, spec)
   local result = {}
 
   for group, opts in pairs(template) do
     if type(opts) == 'table' then
       local new = {}
+
       for key, value in pairs(opts) do
-        if type(value) == 'string' then
-          new[key] = parse_string(value, spec)
-        elseif type(value) == 'number' then
-          new[key] = value
-        end
+        new[key] = parse_keypath(value, spec)
       end
+
       result[group] = new
     else
-      result[group] = parse_string(opts, spec)
+      result[group] = parse_keypath(opts, spec)
     end
   end
 
@@ -56,7 +57,7 @@ function M.parse_template_str(template, spec)
   return (
     template:gsub('($%b{})', function(w)
       local path = w:sub(3, -2)
-      local value = get_path(spec, path) or w
+      local value = get_keypath(spec, path) or w
       if type(value) == 'table' then
         return value.base and value.base or value
       else