refactor(compiler): resolve output before args

Problem: presets that need the output path in their args function
(markdown, github) had to recompute it inline, duplicating the same
gsub expression already in the output field.

Solution: resolve output_file first in M.compile, then extend ctx with
output = output_file into a resolved_ctx before evaluating args and cwd.
Presets can now reference ctx.output directly. Add output? to the
preview.Context type annotation.

lua/preview/init.lua

@@ -19,6 +19,7 @@
 ---@field file string
 ---@field root string
 ---@field ft string
+---@field output? string
 
 ---@class preview.Diagnostic
 ---@field lnum integer