Face primacy and palettes

[tecosaur]

These changes give Faces primacy, replacing the use of Symbols to name faces. This change is capitalised on to address several long-standing pain points with face/style management in StyledStrings.

The supplants #99 and #110, and perhaps also #100.

This closes #87, and should also help with #61.

Motivation

The current system has several issues:

• Piracy with dispatch on AnnotatedString printing
• No way for other packages to define annotation types with custom display
• Any value type in annotations
• No namespacing for faces (all faces in one global dictionary)
• Long package prefixes required (e.g. REPL_History_search_unselected)
• Silent failures with misspelled face names (styled"{bluue:text}" renders unstyled)
• Mixed use of Faces and Symbols throughout the API

Solution

Parameterised annotation values: AnnotatedString{S} becomes AnnotatedString{S, V}, allowing dispatch on the annotation value type without piracy (see: JuliaLang/julia#60527).

Face primacy: Named faces are now referenced by Face objects rather than Symbols. The face"" macro provides convenient access (face"red" instead of :red). Faces are hybrid-interned: created at compile time, registered at runtime.

Palettes: Three new macros for module-scoped face definitions:

• @defpalette! - define a set of named faces
• @registerpalette! - register faces at runtime (in __init__)
• @usepalettes! - import faces from other modules

Face remapping: remapfaces substitutes faces at string construction time, complementing the existing withfaces which operates at display time.

Example

# Before
const FACES = (:MyPkg_header => Face(foreground=:blue, weight=:bold),)
__init__() = foreach(addface!, FACES)
styled"{MyPkg_header:Title}"

# After  
@defpalette! begin
    header = Face(foreground = blue, weight = :bold)
end
__init__() = @registerpalette!
styled"{header:Title}"

Breaking changes

None. The old Symbol-based API remains supported with deprecation warnings.