[TemplateLiteral] Basic lexer implementation

[croraf]

Very basic template literal implementation, of lexer only.

[codecov]

Codecov Report

Merging #455 into master will increase coverage by 0.10%.
The diff coverage is 85.18%.

@@            Coverage Diff             @@
##           master     #455      +/-   ##
==========================================
+ Coverage   66.83%   66.93%   +0.10%     
==========================================
  Files         127      127              
  Lines        8888     8915      +27     
==========================================
+ Hits         5940     5967      +27     
  Misses       2948     2948              

Impacted Files	Coverage Δ
boa/src/syntax/ast/token.rs	45.00% <66.66%> (+0.67%)	⬆️
boa/src/syntax/lexer/mod.rs	56.09% <75.00%> (+1.57%)	⬆️
boa/src/syntax/lexer/tests.rs	99.66% <100.00%> (+0.01%)	⬆️

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[Razican]

I think this is ok for now, in the future it would be nice to check internal expressions, but not sure if this should be done by the lexer or by the parser.

I added a small comment in order to add some documentation for the token.