fix: set default mainfont to Noto Sans CJK SC

The old default lmodern font has some characters missing, like greek
character 'mu' μ.

resolved #5.

latex/preamble.tex

@@ -7,6 +7,9 @@
 
 \usepackage{gbt7714}
 
+\usepackage{fontspec}
+\setmainfont{Noto Sans CJK SC}
+
 \renewcommand{\textfraction}{0.05}
 \renewcommand{\topfraction}{0.8}
 \renewcommand{\bottomfraction}{0.8}