perf(packages/codegen): add functions for writing/marking unnamed mappings (#25975)

Small optimization to sourcemap builds.

Add functions for writing with a source mapping, where it's known from what's being written that the mapping cannot need a name, because `node.name` is never a string, and change all call sites to use the appropriate function.

This allows removing 2 checks from ``recordSourceMapping``:

- `typeof node.name === "string"` - because that's now statically known.
- `printedName.length > 0` - because identifiers cannot be length.

All `write` and `markMap` functions take a `NamedMappableNode` or `UnnamedMappableNode`, which makes it a type error to call the wrong function.

`markMapNamed` becomes dead code, and is removed.