In order to support source mapping of templates, we need
to be able to tokenize the template in its original context.
When the template is defined inline as a JavaScript string
in a TS/JS source file, the tokenizer must be able to handle
string escape sequences, such as `\n` and `\"` as they
appear in the original source file.
This commit teaches the lexer how to unescape these
sequences, but only when the `escapedString` option is
set to true. Otherwise there is no change to the tokenizing
behaviour.
PR Close#28055
Pete Bacon Darwin
Jason Aden