rewriter: allow setting values regardless of the previous type

[joukewitteveen]

Alterations to kwargs come in two flavors:

1. Remove/replace values (delete/set)
2. Modify values (add/remove)

Only for the second flavor do we care about the type of any existing value.

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This is a minor rewrite of the rewriter. When merged, #12277 will need to be rebased.

Changes:

• The type modification classes no longer have a set_value method.
• The new_node and supported_nodes methods of the type modification classes are now classmethods.
• The new_node method of the type modification classes is now public and can take an initial value.
• When a variable is too complex to modify, the action is actually skipped.

Conceptually, the mutability of kwargs now follows Python variable assignment. You don't change a value by reassigning to a variable name. Instead, this replaces the old value.

When merged, it will be possible to do meson rewrite kwargs set project '/' version '1.0.0' also with a build file like

project('example',
  version : run_command('date', '+%s').stdout().strip()))

which makes the technique suggested in #688 (comment) possible.