Fix lexer span panic with carriage return

[vgel]

Switch to treating bare carriage return as an OS9-style newline, like v8.
Fixes #771

• In Cursor::next_char, when we get a '\r', we now try to consume a '\n', and regardless of whether we find one we advance a newline. This has the effect of treating all of ["\r\n", "\n", "\r"] as identical.
• Removed Cursor::carriage_return, as it's now unused.

[codecov]

Codecov Report

Merging #799 into master will increase coverage by 0.04%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master     #799      +/-   ##
==========================================
+ Coverage   59.21%   59.26%   +0.04%     
==========================================
  Files         155      155              
  Lines        9908     9924      +16     
==========================================
+ Hits         5867     5881      +14     
- Misses       4041     4043       +2     

Impacted Files	Coverage Δ
boa/src/syntax/lexer/cursor.rs	60.68% <75.00%> (-1.71%)	⬇️
boa/src/builtins/error/mod.rs	66.66% <0.00%> (+23.18%)	⬆️

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[Lan2u]

Spec reference https://tc39.es/ecma262/#sec-line-terminators