The lexer for a regex terminating character doesn't handle escapes correctly

[Ben-Steele]

JSONata version: 2.0.2
node version: 16.18.0

The Regex lexer does not properly determine if the terminating "/" of a regex is escaped. It does not consider escaped escape characters.
example:
jsonata expression - $match("test escape \\", /\\/)
result - No terminating / in regular expression
expected result - { match: "\", index: 12, groups: []}

See proposed fix in this PR: https://github.com/jsonata-js/jsonata/pull/623/files

[andrew-coleman]

Resolved by #623

Many thanks.