Add support to determine global variable values based on source tree

[hendrikvanantwerpen]

JavaScript and TypeScript use PROJECT_NAME and FILE_PATH variables, which are supposed to be inferred from package.json or tsconfig.json locations. This inference process is currently implemented in another process, making testing this behavior cumbersome: one has to provide these variables explicitly in the tests, for each file.

Language configurations should be able to provide a custom procedure to determine the value for global variables for source files.

[hendrikvanantwerpen]

Idea: TreeAnalyzers

Similar to FileAnalyzer we define a TreeAnalyzer trait which can be implemented for languages. A tree analyzer takes the source listing and produces a mapping from source files to global variables. These global variables are passed to the file analyzer.

A nice effect of this is that the source file list argument can be removed from the FileAnalyzer trait, at which point a StackGraphLanguage implements FileAnalyzer and everything becomes more uniform.

As part of this, we could/should provide a trie-like datastructure that allows us to set globals on directories as well as files, and returns all globals in effect for a source file. For example, set the PROJECT_NAME variable on the project root, and the FILE_PATH on the file, and when asked for the globals of a file in that directory, return both PROJECT_NAME and FILE_PATH values.