Foreach completion

[phil-nelson]

This adds some basic support for completing foreach keys/values. For a complete list of what works have a look at the test, but I believe most "normal" cases work.

Limitations:

• Using list() for values is unsupported. This isn't straightforward, and as far as I can tell we don't support a straight up list() (outside a foreach) for completion
• No support for iterating public object properties
• No support for \Traversable objects

I think implementing these limitations is probably more significant work than what's here. I think the cases that work are probably the most used/popular, so even given these limitations I think this would be worth having. We can work on the missing bits in the future.

Closes #200

[codecov]

Codecov Report

Merging #551 into master will increase coverage by 0.64%.
The diff coverage is 92.85%.

@@             Coverage Diff             @@
##             master    #551      +/-   ##
===========================================
+ Coverage     80.86%   81.5%   +0.64%     
- Complexity      877     898      +21     
===========================================
  Files            61      61              
  Lines          2028    2055      +27     
===========================================
+ Hits           1640    1675      +35     
+ Misses          388     380       -8

Impacted Files	Coverage Δ	Complexity Δ
src/Server/TextDocument.php	75.37% <100%> (+0.18%)	56 <0> (ø)	⬇️
src/CompletionProvider.php	94.51% <100%> (+0.17%)	108 <0> (+6)	⬆️
src/DefinitionResolver.php	86.69% <90.9%> (+1.64%)	318 <0> (+15)	⬆️
src/Indexer.php	60.75% <0%> (+2.53%)	18% <0%> (ø)	⬇️

[jens1o]

code style; no brackets at the end.

[andreymrt]

@phil-nelson can you please take a look at #561 could be related to your PR on foreach completion.
Following code snippet makes the lang server throw an exception

<?php
foreach($Calendar as $day)
	echo $day->Date->format("D");

While, this one is fine:

<?php
foreach($Calendar as $day)
	echo $day->format("D");