Add command that updates existing attributes with a select

[kisabaka]

Buildtools PR checklist

• The code in this PR is covered by unit/integration tests.
• I have tested these changes and provide testing instructions below.
• I have either responded to, or resolved all Gemini comments on the PR.
• I have read Google Eng Practices on Small Changes, this PR either follows these guidelines or the description provides reasoning for why they can not be followed.

Description

Adds a command to update attributes with a select.

buildozer 'add_select deps @platforms//os:linux :linux_bar' :foo

E.g
Before:

cc_library(
  name = "foo",
  deps = [
    ":bar",
  ]
)

After:

cc_library(
  name = "foo",
  deps = [
    ":bar",
  ] + select({
    "@platforms//os:linux": [":linux_foo"],
    "//conditions:default": [],
  }),
)

(optional) These changes were tested using the following steps

Steps to reproduce / test the PR changes (if anything besides unit tests).
bazel test //...
go test ./edit/...

[gemini-code-assist]

Code Review

This pull request introduces the add_select command to buildozer, enabling the addition or update of values within select expressions for both list and scalar attributes. The implementation includes helper functions to manage existing select blocks, handle concatenation with plain lists, and ensure default conditions are present. Feedback was provided to maintain consistency in list sorting by utilizing the attributeMustNotBeSorted check when adding values to list attributes.