[boa context] Minor refactor and rename in eval() method

[croraf]

Some terms were really wrong, like "parser_expr" or "Ok(expr)".

I also made some opinionated minor modification (like put the private method with one statement inline, and split parsing and execution into two statements).

[codecov]

Codecov Report

Merging #841 into master will decrease coverage by 0.01%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master     #841      +/-   ##
==========================================
- Coverage   59.37%   59.36%   -0.02%     
==========================================
  Files         157      157              
  Lines       10036    10035       -1     
==========================================
- Hits         5959     5957       -2     
- Misses       4077     4078       +1     

Impacted Files	Coverage Δ
boa/src/context.rs	66.51% <60.00%> (-0.61%)	⬇️

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