Disallow unbalanced bidirectional unicode

[c42f]

Disallow unbalanced Unicode bidirectional formatting directives within strings and comments, to mitigate the "trojan source" vulnerability https://www.trojansource.codes

See also https://github.com/JuliaLang/julia/pull/42918

Fix #242 Closes #270

[codecov[bot]]

Codecov Report

Merging #288 (4b427da) into main (6e3782f) will increase coverage by 0.02%. The diff coverage is 96.77%.

@@            Coverage Diff             @@
##             main     #288      +/-   ##
==========================================
+ Coverage   96.73%   96.75%   +0.02%     
==========================================
  Files          14       14              
  Lines        4038     4071      +33     
==========================================
+ Hits         3906     3939      +33     
  Misses        132      132              

Impacted Files	Coverage Δ
src/kinds.jl	80.59% <ø> (ø)
src/parser.jl	98.26% <75.00%> (-0.06%)	:arrow_down:
src/tokenize.jl	99.06% <98.21%> (+0.17%)	:arrow_up:
src/parse_stream.jl	96.42% <100.00%> (+0.01%)	:arrow_up: