Fix portability of make rules to build documentation

Support for "$<" automatic variable in non-suffix rules is a GNU Make
extension: don't use it.
JavaScript: fix parsing of files starting with a shebang

Closes #1016.
Add a Rust ctags generator
Added the Rust filetype
Add Rust ctag tests
Add Rust ctag test outputs
Add Rust tests to the makefile
Use :: as the context separator for Rust
Fix warnings and a few bugs uncovered by them
Import the Rust lexer from Scintilla
Integrate the Rust lexer with the rest of Geany's functionality
Enable highlighlighting of user types in Rust documents
Detection of Rust comment styling was buggy
Add shebang filetype detection for Rust
Add an empty default case to a switch
Sort the Rust keywords
Fix the explanation of what scanComments looks for
Use 'Modules' instead of 'Module' for the Rust module item
Make Rust ctag tests valid Rust code
Clean up Rust ctag tests and make them more comprehensive