Add support for Swift 5.5's concurrency features (JohnSundell#126)

This patch adds syntax highlighting support for the new concurrency keywords
introduced in Swift 5.5 - `actor`, `async`, and `await`. It also includes supporting
changes to make sure that usages of these new features/keywords are highlighted
correctly, and to protect against regressions within existing Splash-highlighted code.

Improve highlighting of comments when placed next to symbols (JohnSun…

…dell#120)

Especially for multi-line comments placed next to generic type lists.

Fix build product path to work on case sensitive and insensitive file…

… systems (JohnSundell#116)

Correctly highlight subclasses inheriting from a generic superclass (J…

…ohnSundell#108)

This change fixes syntax highlighting in situations when a subclass is
inheriting from a superclass that’s generic.

Don’t treat accessing properties within switch statements as calls (J…

…ohnSundell#103)

This patch fixes syntax highlighting for when a property is being switched
on, which previously would be treated as a function call with trailing
closure syntax.

Fix Swift 5.2 compiler crash (JohnSundell#99)

Added Comment component test with a number (JohnSundell#90)

Add support for the “prefix” keyword (JohnSundell#91)

This patch makes Splash correctly highlight the `prefix` keyword, which
can only appear before `func`, so a special case was added for it (for now).

Support highlighting custom compiler directives (JohnSundell#88)

Add support for highlighting the `#warning` and `#error` directives.

Fix highlighting for comments starting with a delimiter (JohnSundell#82)

This patch fixes syntax highlighting for any comments that begin with
a delimiter, which would previously prevent the comment’s leading token
from being parsed correctly. The fix is to prevent slashes to be merged
with any other delimiter, except for other slashes and asterisks.