Ignore diagnostics by steep:ignore comment

[soutaro]

This PR implements ignoring diagnostics by embedded comments.

# Ignoring a range of lines

# steep:ignore:start

foo()      # NoMethod is detected, but ignored

# steep:ignore:end


# Ignoring a specific line

foo() # steep:ignore
foo() # steep:ignore NoMethod

Possible enhancements:

• Reporting unused ignore comments to be deleted later
• Adding completion

(This is also proposed at #964, but the implementation is slightly different. It generates errors, but filters out at later step. It also reports unbalanced start-end comments.)