Make it easier to modify behavior of `parse` / `tryparse` / `xparse2`

[Drvi]

Currently, users are expected to provide a new Options object when they need to change the default behavior from Parsers.OPTIONS. However, the main constructor for Options contains default values which are good for xparse, not necessarily for xparse2 -- for example, the delim field is set to , in that constructor while we want it to be nothing for xparse2. We should provide an easy way for users to change parsing params for xparse2 which preserves the default values we see in Parsers.OPTIONS.

[nickrobinson251]

is xparse2 public? i could only find FixedPointDecimals.jl using it, fwiw https://github.com/JuliaMath/FixedPointDecimals.jl/blob/0e042148d473e1ef304302cbde049e71d26b9bd0/src/parse.jl#L220

[Drvi]

I'm not sure, but it is called by both Parsers.tryparse and Parsers.parse, which are public