Add new SourceFileService

[knutwannheden]

The service implements the methods computeWeight() and newVisitor(). This is then in turn used by SourceFile#getWeight() with the idea that it will automatically use a visitor that doesn't need to be adapted using TreeVisitorAdapter, which behind the scenes dynamically generates a new class.

[knutwannheden]

Closing for now.

[JLLeitschuh]

Use cases for services:

• Control Flow analysis will be, likely, unique per-language as the characteristics around dataflow are unique based upon language.
• Custom logic that surrounds printing the AST. For example:
  • Control Flow Analysis: https://github.com/openrewrite/rewrite-analysis/blob/main/src/main/java/org/openrewrite/analysis/controlflow/ControlFlowJavaPrinter.java
  • Detecting AST elements by line/column number: Support Locating AST elements by Line/Column rewrite-analysis#41

[knutwannheden]

For control flow analysis we would likely add another dedicated service. We already have a few services, but for now we have limited this mechanism to J and extending languages. But I think it is soon time to pull this up into the core.

@JLLeitschuh Thanks for your use cases.