Extract f-string prefix as a separate token

[tancnle]

The f-string prefix was extracted together with the text as one token. This makes syntax highlight less flexible.

This change will:

• Extract f prefix as String::Affix token, similar to how it is done in Pygments.
• Add styles for String::Affix in all themes (mirror the style of Keyword)

This fixes #1634.

Before

[1] pry(main)> require './lib/rouge'
true
[2] pry(main)> lexer = Rouge::Lexers::Python.new
#<Rouge::Lexers::Python:0x00007fadc66979b0 @options={}, @debug=false>
[3] pry(main)> lexer.lex("foo = f\"test: {t}\"\n").to_a
[
    [0] [
        [0] <Token Name> < Rouge::Token,
        [1] "foo"
    ],
    [1] [
        [0] <Token Text> < Rouge::Token,
        [1] " "
    ],
    [2] [
        [0] <Token Operator> < Rouge::Token,
        [1] "="
    ],
    [3] [
        [0] <Token Text> < Rouge::Token,
        [1] " "
    ],
    [4] [
        [0] <Token Literal.String> < Rouge::Token::Tokens::Literal,
        [1] "f\"test: "
    ],
    [5] [
        [0] <Token Literal.String.Interpol> < Rouge::Token::Tokens::Literal::String,
        [1] "{"
    ],
    [6] [
        [0] <Token Name> < Rouge::Token,
        [1] "t"
    ],
    [7] [
        [0] <Token Literal.String.Interpol> < Rouge::Token::Tokens::Literal::String,
        [1] "}"
    ],
    [8] [
        [0] <Token Literal.String> < Rouge::Token::Tokens::Literal,
        [1] "\""
    ],
    [9] [
        [0] <Token Text> < Rouge::Token,
        [1] "\n"
    ]
]

After

[1] pry(main)> require './lib/rouge'
true
[2] pry(main)> lexer = Rouge::Lexers::Python.new
#<Rouge::Lexers::Python:0x00007fe355697bf8 @options={}, @debug=false>
[3] pry(main)> lexer.lex("foo = f\"test: {t}\"\n").to_a
[
    [ 0] [
        [0] <Token Name> < Rouge::Token,
        [1] "foo"
    ],
    [ 1] [
        [0] <Token Text> < Rouge::Token,
        [1] " "
    ],
    [ 2] [
        [0] <Token Operator> < Rouge::Token,
        [1] "="
    ],
    [ 3] [
        [0] <Token Text> < Rouge::Token,
        [1] " "
    ],
    [ 4] [
        [0] <Token Literal.String.Affix> < Rouge::Token::Tokens::Literal::String,
        [1] "f"
    ],
    [ 5] [
        [0] <Token Literal.String> < Rouge::Token::Tokens::Literal,
        [1] "\"test: "
    ],
    [ 6] [
        [0] <Token Literal.String.Interpol> < Rouge::Token::Tokens::Literal::String,
        [1] "{"
    ],
    [ 7] [
        [0] <Token Name> < Rouge::Token,
        [1] "t"
    ],
    [ 8] [
        [0] <Token Literal.String.Interpol> < Rouge::Token::Tokens::Literal::String,
        [1] "}"
    ],
    [ 9] [
        [0] <Token Literal.String> < Rouge::Token::Tokens::Literal,
        [1] "\""
    ],
    [10] [
        [0] <Token Text> < Rouge::Token,
        [1] "\n"
    ]
]

Before (base16-dark)	After (base16-dark)

[tancnle]

It seems like we already have the f-string samples.

[tancnle]

I think we need to add the new style to themes as well. 🤔

[pyrmont]

@tancnle Thanks for adding this! Looks good!