Add parsing of context reference blocks together with context declarations

Add functions to the TokenToBlockParser which can be generally useful:

HandleNonCodeTokens creates NonCode (i.e. whitespace and comment)
blocks. The LRM allows these blocks basically everywhere.
Note: Delimited comments are strictly speaking not separators
according to the LRM (§15.3)!

ReparseFromTokenMarker allows to iterate over a set of tokens a second
time. This is useful if you cannot decide the block type based on a
single token. `Context.StartBlock` searches ahead (without modifying
tokens or adding blocks) until it can decide whether the `context`
keyword is used in a context declaration or a context reference.

Additionally, some changes are made to satisfy the static type checker.

Author: Sebastian Kaupper

.gitignore

@@ -25,3 +25,4 @@ coverage.xml
 /.idea/workspace.xml
 
 pyVHDLParser/Sven
+venv/