Add power symbols (Proposal 5)

[T0mstone]

This PR implements Proposal 5 (iteration 1) from the Symbols Proposal Document, although I tweaked it a little.

In particular, my tweaks are based on https://en.wikipedia.org/wiki/Power_symbol#Unicode,
adding power.off with value U+2B58 HEAVY CIRCLE and changing the plain power to power.standby (since that's what it represents).

IIUC, if the symbol doesn't have a character, it defaults to its first variant, right? That way, power without modifiers is also what it is in the proposal.

[MDLC01]

I'm unsure those symbols should be added. And I'm even more unsure about power.sleep, which comes from a different set (IEEE 1621-2004, while the others are from ISO 7000:2012), and is much less widely used.

This PR implements Proposal 5 (iteration 1) from the Symbols Proposal Document, although I tweaked it a little.

I edited the document to create a second iteration of the proposal, which should match this PR.

adding power.off with value U+2B58 HEAVY CIRCLE and changing the plain power to power.standby (since that's what it represents).

Additionally, it U+2B58 has the informative alias "power off symbol."

IIUC, if the symbol doesn't have a character, it defaults to its first variant, right?

Right.

[mkorje]

I don't really see any issue adding these symbols. Even though power.sleep comes from a different set, they were all added together to Unicode.

[T0mstone]

I'll repeat here what I've said on Discord:

[It's true that] I don't see a use for them in written documents, but they're really universally recognized symbols, so I'd say adding them doesn't hurt.