ANSI dim text styling

[mattmorgis]

This PR adds complete support for ANSI dim text styling. Previously, prompt-toolkit would strip or ignore ANSI dim formatting when processing text from Rich or other libraries that use \x1b[2m escape sequences. This meant that dim text would appear normal instead of faded/dimmed as intended.

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You can test the dim functionality with this script:

from prompt_toolkit.formatted_text.ansi import ANSI
from prompt_toolkit import print_formatted_text

# Test basic dim
print_formatted_text(ANSI("\x1b[2mThis text should appear dim\x1b[0m"))

# Test dim with color
print_formatted_text(ANSI("\x1b[2;31mDim red text\x1b[0m"))

# Test bold + dim combination
print_formatted_text(ANSI("\x1b[1;2;34mBold dim blue text\x1b[0m"))

# Visual comparison (side by side)
print_formatted_text(ANSI("Normal vs \x1b[2mDim\x1b[0m"))

# Test dim reset
print_formatted_text(ANSI("\x1b[1;2mBold+dim\x1b[22mnormal\x1b[0m"))

Using Prompt-Toolkit native styles:

from prompt_toolkit import print_formatted_text
from prompt_toolkit.formatted_text import FormattedText
from prompt_toolkit.styles import Style

style = Style.from_dict({
    'dim-text': 'dim',
    'dim-red': 'dim ansired',
    'bold-dim': 'bold dim',
    'dim-blue': 'dim ansiblue',
})

print_formatted_text(FormattedText([
    ('', 'Normal text '),
    ('class:dim-text', 'dim text '),
    ('class:dim-red', 'dim red '),
    ('class:bold-dim', 'bold dim'),
]), style=style)

# Inline styles also work
print_formatted_text(FormattedText([
    ('dim', 'This is dim '),
    ('dim ansired', 'this is dim red '),
    ('bold dim ansiblue', 'this is bold dim blue'),
]))

It also matches Rich's [dim] formatting exactly:

import io
from rich.console import Console
from rich.text import Text
from prompt_toolkit.formatted_text.ansi import ANSI
from prompt_toolkit import print_formatted_text

console = Console(file=io.StringIO(), force_terminal=True)
console.print("This is [dim]dimmed text[/dim]")
rich_output = console.file.getvalue()

# dim formatting is preserved
print_formatted_text(ANSI(rich_output))