Add Greek characters to list of acceptable prompt-symbols

[Alhadis]

Description of the Change

This PR enables Greek characters to be tokenised as prompt-string symbols in shell-session snippets:

λ echo $foo $bar
Foo
Bar
λ man apm
No manual entry for apm
λ man atom
No manual entry for atom.
Pathetic.
λ quit

As the above snippet illustrates, the lines beginning with λ aren't being scoped as command lines.

Compare with $:

$ echo $foo $bar
Foo
Bar
$ man apm
…

Note:
My shell's PS1 isn't an isolated case. Lambda calculus, functional programming, and probably Half-life have helped propagate spiffy-looking Greek things in any notation intended to look mathematical or programming-related.

Mocha's homepage features screenshots which also show λ in use.

Alternate Designs

I thought to include \p{S} in the pattern-block, which matches anything with a Unicode Symbol property.
The scope was too broad, and compatibility issues between Oniguruma and PCRE reared its ugly head.

Thankfully, both engines agree on the syntax for matching Unicode script-properties.

Benefits

Improved highlighting on GitHub. Specifically in README files, where example usage often takes the form of a terminal session's output.

Possible Drawbacks / Applicable Issues

None I can think of.

[Alhadis]

/cc @50Wliu, because I know he loves SourcePawn and Half-Life and stuff. =)

[winstliu]

Ha!