Fix lexing escapes in string literal and minor refactor (#1079)

* Refactor StringLiteral

* Fix octal escape in string literal

* Add tests

* Fix zero escape

* Fix zero escape lookahead

* Rename variables

* Rename helper functions

* Refactor match arms

* Fix escape line terminator sequence

* Fix single character escape

* Fix escape followed by unicode char

* Add NonOctalDecimalEscapeSequence

* Fix comment

* Refactor

* Modify error message

* Add tests

* Rename tests

* Add test for error

* Add comments for unsafe bytes to str

* Update boa/src/syntax/lexer/string.rs

Co-authored-by: tofpie <[email protected]>

* Minor refactor

* Remove unsafe bytes to str

* Fix panic when reading invalid utf-8 chars

Co-authored-by: tofpie <[email protected]>