luamath parser doesn't support /pgf/number format/read comma as period

[muzimuzhi]

Brief outline of the bug

The luamath math expression parser always uses period . as decimal point, thus it fails (silently) when being used to parse a single number with setting /pgf/number format/read comma as period=true.

pgf/tex/generic/pgf/libraries/luamath/pgf/luamath/parser.lua

Lines 47 to 49 in 425b03d

	-- Valid positive decimals are |xxx.xxx|, |.xxx| and |xxx.|
	local positive_integer_or_decimal_pattern = positive_integer_pattern * ( P(".") * one_digit_pattern^0)^-1 +
	(P(".") * one_digit_pattern^1)

This makes luamath not a drop-in replacement for pgfmath plus fpu library, and result in inconsistent behaviors under LuaTeX and non-LuaTeX engines, see for example pgf-tikz/pgfplots#452.

It seems luamath should re-bind its own \pgfmathfloatparsenumber.

Minimal working example (MWE)

% !TeX program = lualatex
\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{luamath}

\begin{document}
\pgfset{number format/read comma as period}

\pgfmathfloatparsenumber{1234,56}\pgfmathresult

\pgfset{luamath/parser and computation}
\pgfmathfloatparsenumber{1234,56}\pgfmathresult % works because of the fallback

\directlua{pgfluamathparser.texCallParser("1234,56"), ,true}\pgfmathresult
\end{document}

The direct call of pgfluamathparser.texCallParser("1234,56"), ,true throws error

))[\directlua]:1: syntax error near ','.
l.21 ...amathparser.texCallParser("1234,56"), ,true}
                                                  \pgfmathresult
The lua interpreter ran into a problem, so the
remainder of this lua chunk will be ignored.

The logic of fallback is mentioned in lines 537--538 and implemented in lines 549--544 below

pgf/tex/generic/pgf/libraries/luamath/pgflibraryluamath.code.tex

Lines 533 to 555 in 425b03d

	% Parses the math expression and defines \pgfmathresult and
	% \ifpgfmathunitsdeclared to contain the result.
	%
	% If \ifpgfluamathparseractive then the result is also assigned to
	% \pgfmathresult. Furthermore, if the expression cannot be evaluated (because LUA does not support
	% it), it will be evaluated by mean of TeX as fallback.
	%
	% @see \ifpgfluamathusedTeXfallback
	% @see \ifpgfluamathshowerrormessage
	\def\pgfluamathparse#1{%
	\pgfluamathusedTeXfallbackfalse
	\pgfutil@directlua{pgfluamathparser.texCallParser(
	"\pgfutil@luaescapestring{#1}",
	\pgfluamath@outputformat@choice,
	\ifpgfluamathshowerrormessage true\else false\fi)%
	}%
	\ifx\pgfmathresult\pgfutil@empty
	\ifpgfluamathenableTeXfallback
	\pgfluamathusedTeXfallbacktrue
	\pgfluamath@pgfmathparse{#1}%
	\fi
	\fi
	}%