fix(grammar): support single-quoted keys in compound pairs

[AnonymoDGH]

Related to #63

Investigation of #63

The exact payload from the issue parses correctly on current master — single-quoted string values were already fixed by #53 (merged 2025-12-22). The reporter's stack trace (index.js:94, offset: 42, token: { value: 's' }) matches the published npm package 2.0.4 (2023-06-03), which predates that fix. I verified this by installing mojangson@2.0.4 and feeding it the issue's payload: it fails with the identical error, while master parses it fine.

So the main fix for #63 is already on master — it just hasn't been published (npm latest is still 2.0.4, master is 2.1.0). Publishing 2.1.0 would resolve the report.

What this PR fixes

While verifying, I found a genuine remaining gap: single-quoted keys still fail, even though Minecraft SNBT allows them:

mojangson.parse("{'key':'value'}")  // throws: offset 1, token "'"

The PAIR rule only accepted STRING as a key, and STRING's unquoted branch excludes '. This adds a SINGLE_QUOTED_STRING alternative to PAIR, so single-quoted keys work everywhere double-quoted keys do (including keys with spaces, colons, and escaped quotes).

Verified

• {'key':'value'}, {'key':"value"}, {'a':1,"b":2}, {'my key':'v'}, {'minecraft:id':'x'}, nested {'outer':{'inner':'v'}} all parse correctly.
• No regressions: unquoted keys, double-quoted keys, and single-quoted values all still work.
• npm test: 140 passing (standard lint + mocha), including 6 new single-quoted-key cases.

Note: grammar.js is a gitignored build artifact regenerated from grammar.ne via nearleyc (prepublish), so only grammar.ne is changed here.