web: a few glyphs are missed in the default font

[hermet]

It's not visible on the thorvg viewer:
sample: https://github.com/thorvg/thorvg/blob/main/examples/resources/lottie/expressions/24243.json

The default font might not include numeric characters.

[hermet]

glyphs works but still + is missing.

[hermet]

+ @tinyjin

[tinyjin]

@hermet
Currently default font only contains A-Z, a-z, 0-9, we can add more to resolve this.
Only '+' is enough? For special symbols, we can add more general characters, suggesting '+', '-', '.', '=', '(', ')'

It'd be nicer if we can confirm together

[hermet]

'+', '-', '.', '=', '(', ')'
I agree for a quick-solver.