Add ability to only run annotation processing/only run compilation

[ascopes]

Adds a new option to JctCompiler that enables the user to specify the
compiler mode to run under.

By default, nothing is changed, but the user may opt to specify the
ANNOTATION_PROCESSING_ONLY option which will pass -proc:only to
Javac, or to specify COMPILATION_ONLY which will pass -proc:none
to Javac.

[codecov-commenter]

Codecov Report

Merging #265 (12a6bf2) into main (e5aca93) will increase coverage by 0.17%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #265      +/-   ##
==========================================
+ Coverage   70.89%   71.07%   +0.17%     
==========================================
  Files          72       73       +1     
  Lines        2536     2551      +15     
  Branches      237      237              
==========================================
+ Hits         1798     1813      +15     
  Misses        671      671              
  Partials       67       67              

Impacted Files	Coverage Δ
...hub/ascopes/jct/compilers/AbstractJctCompiler.java	100.00% <100.00%> (ø)
.../github/ascopes/jct/compilers/CompilationMode.java	100.00% <100.00%> (ø)
...a/io/github/ascopes/jct/compilers/JctCompiler.java	100.00% <100.00%> (ø)
...s/jct/compilers/javac/JavacJctFlagBuilderImpl.java	100.00% <100.00%> (ø)

[github-actions]

Test Results

  1 462 files  +  14    1 462 suites  +14   0s ⏱️ ±0s
18 172 tests +134  18 172 ✔️ +134  0 💤 ±0  0 ❌ ±0 
18 458 runs  +134  18 458 ✔️ +134  0 💤 ±0  0 ❌ ±0 

Results for commit 12a6bf2. ± Comparison against base commit e5aca93.