Update checking of $this usage to only error in static methods.

[MaartenStaa]

This rolls back a few of the changes in #528 to address the concerns in #536. It will now only produce an error in static methods, and leave usages of $this in the root and in closures alone.

[codecov]

Codecov Report

Merging #545 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@ Coverage Diff @@ ## master #545 +/- ## ============================================ - Coverage 79.38% 79.36% -0.02%  + Complexity 837 835 -2  ============================================ Files 56 56 Lines 1979 1900 -79 ============================================ - Hits 1571 1508 -63  + Misses 408 392 -16

Impacted Files	Coverage Δ	Complexity Δ
src/TreeAnalyzer.php	94.28% <100%> (-0.36%)	52 <0> (-2)
src/Client/XCache.php	33.33% <0%> (-9.53%)	3% <0%> (ø)
src/Client/Window.php	66.66% <0%> (-4.77%)	3% <0%> (ø)
src/Protocol/Position.php	53.84% <0%> (-3.3%)	6% <0%> (ø)
src/Server/Workspace.php	38.46% <0%> (-2.28%)	23% <0%> (ø)
src/Cache/FileSystemCache.php	25% <0%> (-2.28%)	8% <0%> (ø)
src/Protocol/Range.php	80% <0%> (-1.82%)	4% <0%> (ø)
src/Server/TextDocument.php	74.21% <0%> (-0.79%)	55% <0%> (ø)
src/PhpDocument.php	79.31% <0%> (-0.69%)	24% <0%> (ø)
src/Index/Index.php	69.81% <0%> (-0.69%)	22% <0%> (ø)
... and 40 more

[jens1o]

Personally, I'd prefer the approach in #543