squirrel3: Allow floating number literal starting with dot for lexer

Squirrel language does not support floating number literals beginning
with a dot, e.g. ".12", ".001" or ".5e3", although C language family
support them.  This is an attempt to modify Squirrel lexical scanner
to accept those literals.

I think this change won't affect existing syntax since a token
beginning with a dot and a digit became an immediate error.
squirrel3: Allow exponential notation without a dot

Squirrel language does not support floating number literals with
scientific notation (with exponential part) but without a dot, e.g.
"12e3" or "1e5".  This specification seems unreasonable and almost
a bug.

In a space simulator like ours, astronomic values often occur in
scripts which require compact scientific notations.