Draft: POC - add code tasks

What does this MR do and why?

Related to &11734 (closed)

A POC of adding code tasks:

• accepts current_file's arguments (filename, selected text, content above and below)
• adds a new EditCode tool useful for doing any code changes
• adds a couple of slash commands for this tool

Can be tested with mutation like this:

mutation {
  aiAction(input: {chat: {content: "/refactor", currentFile: {
    fileName: "test.py",
    selectedText: "
def extract
  lines = content.to_s.lines
  comment_block = []
  trimmed_lines = 0
end
",
    contentAboveCursor: "
module CodeSuggestions
  class InstructionsExtractor
    EMPTY_LINES_LIMIT = 1
    MIN_LINES_OF_CODE = 5
",
  }}}) {
    clientMutationId
    errors
  }
}

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