Allow customizing parameter regex

[amake]

In c4a3c32 the parameter matching regex was changed from

  const regex = /{{(\w|:|[\s-+.,@/\//()?=*_$])+}}/g;

to

  const regex = /\{\{\s*([\p{L}_$][\p{L}\p{N}_.]*)(?::([^}]*))?\s*\}\}/gu;

While I think being stricter about parameter names is fine in theory, this is a big break in compatibility and broke lots of our saved templates.

I actually do need Unicode support, so I wouldn't ask to go back to the old regex. However it might make sense to allow consumers to specify their own regex.

[amake]

I had to vendor this library and modify the regex in order to restore backwards compatibility; the regex I ended up with is:

/\{\{\s*([\p{L}_$][\p{L}\p{N}\s+,@/()?=*_.$-]*)(?::([^}]*))?\s*\}\}/gu

[amake]

It would also be useful to have a function to validate that a parameter name will be matched. (Obviously if the regex is customizable then the validation would depend on the regex)