Implement early errors for non-assignable nodes in assignment

[tofpie]

This Pull Request implements early error for non-assignable nodes in assignment.

It changes the following:

• is_assignable function in Assignment parsing

[codecov]

Codecov Report

Merging #1019 (e5e5e4a) into master (4625c1a) will decrease coverage by 0.00%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #1019      +/-   ##
==========================================
- Coverage   60.10%   60.10%   -0.01%     
==========================================
  Files         167      167              
  Lines       11142    11143       +1     
==========================================
  Hits         6697     6697              
- Misses       4445     4446       +1     

Impacted Files	Coverage Δ
boa/src/syntax/parser/expression/assignment/mod.rs	50.00% <50.00%> (-0.59%)	⬇️

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[Razican]

Test262 conformance changes:

Test result	master count	PR count	difference
Total	78,493	78,493	0
Passed	20,882	21,107	+225
Ignored	15,585	15,585	0
Failed	42,026	41,801	-225
Panics	388	388	0
Conformance	26.60	26.89	+0.29%

[Razican]

Thanks! Looking good!