Use factory pattern to initialise flag and JavaCompiler objects

[ascopes]

Replace #getFlagBuilder and #getJsr199Compiler with factory pattern in AbstractJctCompiler class and implementations.

[github-actions]

Test Results

  1 387 files  ±    0    1 387 suites  ±0   0s ⏱️ ±0s
18 212 tests  - 108  18 200 ✔️  - 108  12 💤 ±0  0 ❌ ±0 
18 488 runs   - 108  18 476 ✔️  - 108  12 💤 ±0  0 ❌ ±0 

Results for commit 0317070. ± Comparison against base commit 3e0de86.

[codecov-commenter]

Codecov Report

Merging #278 (0317070) into main (3e0de86) will decrease coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #278      +/-   ##
==========================================
- Coverage   71.04%   71.00%   -0.05%     
==========================================
  Files          75       75              
  Lines        2625     2621       -4     
  Branches      239      239              
==========================================
- Hits         1865     1861       -4     
  Misses        693      693              
  Partials       67       67              

Impacted Files	Coverage Δ
...hub/ascopes/jct/compilers/AbstractJctCompiler.java	100.00% <100.00%> (ø)
...ascopes/jct/compilers/impl/JctCompilationImpl.java	100.00% <100.00%> (ø)
...opes/jct/compilers/javac/JavacJctCompilerImpl.java	100.00% <100.00%> (ø)