ANSI terminal themes; culture-specific formatting

[nblumhardt]

Fixes #8, fixes #19.

Theming is implemented by the TemplateTheme class; built-in themes are TemplateTheme.Code, TemplateTheme.Grayscale, and TemplateTheme.Literate, which were lifted directly from Serilog.Sinks.Console.

Themes can be user-specified, and can inherit from a "base" theme:

var melon = new TemplateTheme(TemplateTheme.Literate, new Dictionary<TemplateThemeStyle, string>
{
    // `Information` is dark green in MEL.
    [TemplateThemeStyle.LevelInformation] = "\x1b[38;5;34m",
    [TemplateThemeStyle.String] = "\x1b[38;5;159m",
    [TemplateThemeStyle.Number] = "\x1b[38;5;159m"
});

using var log = new LoggerConfiguration()
    .WriteTo.Console(new ExpressionTemplate(
        "{@l:w4}: {SourceContext}\n" +
        "{#if Scope is not null}" +
        "      {#each s in Scope}=> {s}{#delimit} {#end}\n" +
        "{#end}" +
        "      {@m}\n" +
        "{@x}",
        theme: melon))
    .CreateLogger();

Produces:

(LOL 😁)

Only ANSI escape sequence-based theming is supported (so on Windows, only recent versions of the console will support them, along with Windows Terminal).

Themed message template rendering touched on support for IFormatProvider, so I've rolled in those changes, too.

There's minor API breakage, with optional or nullable IFormatProvider arguments added to the various SerilogExpression factory methods, and optional/nullable TemplateTheme arguments added to the ExpressionTemplate constructors/factory methods.

Fun twist - since the same templating implementation is used for text and JSON output, you can also do this:

Perf impact shouldn't be huge, and since more work is brought back to template "compile" time (there are no runtime dictionary lookups), I'm expecting that theming will be slightly faster here than in Serilog.Sinks.Console, but that's for another day.

Tests are also a bit light, but the code paths covered by themed and un-themed output are identical apart from the behavior implemented in the (tiny) Style and StyleReset structs, so I think I can get away with it 😉

[nblumhardt]

Test for a small bugfix that's also included - the parser was defaulting alignment to 0/Left, so some unnecessary buffering was performed.

[nblumhardt]

Not anticipating that anyone is going to want to/have the time to review this sprawling PR, so sending it out into the wild via -dev so that some feedback can be collected. 👀 welcome, though :-)