Fix parsing of chained unary and unary type operators

[c42f]

• Parsing * <: A doesn't crash the parser
• + <: A now parses correctly as (call-pre + (<:-pre A)).

Fixes part of #186

[codecov[bot]]

Codecov Report

Merging #249 (133fbe5) into main (1515d8f) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #249   +/-   ##
=======================================
  Coverage   96.47%   96.48%           
=======================================
  Files          15       15           
  Lines        3919     3922    +3     
=======================================
+ Hits         3781     3784    +3     
  Misses        138      138           

Impacted Files	Coverage Δ
src/parser.jl	98.25% <100.00%> (+<0.01%)	:arrow_up:

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