Add indent contextmanager

[spenczar]

This adds an indent contextmanager for managing indentation depth. In practice I find this rather ergonomic and helpful. Example usage:

g.P("class Service")
with g.indent():
    g.P("def method(self):")
    with g.indent():
        g.P("x = 4 + 2")
        g.P("return x")
    g.P("")
    g.P("def another_method(self):")
    with g.indent():
        g.P("pass")

This way, indenting and dedenting in the generated code matches the source generator code.

[fischor]

I like it, as the indentation of the code reflects to indentation of the generated output.
Can you rebase it to the current main branch so that we can make sure the CI pipeline stays green?

Thank you for all your PRs!

[spenczar]

Sure, just rebased and force-pushed. Glad to help - thanks for making this package, it's very helpful within its little niche.