Add check for unquoted variable (repoman has it)

[Flowdalic]

Repoman reports, at least some, problematic unquoted variables. Ideally pkgcheck would do the same.

Example

Running pkgcheck
app-emulation/xen-tools
  UnstableOnly: for arches: [ arm, arm64 ], all versions are unstable: [ 4.15.1-r2, 4.15.2, 4.15.2-r1, 4.16.0-r3 ]
  PotentialStable: version 4.15.1-r2: slot(0), stabled arches: [ amd64, x86 ], potentials: [ ~arm, ~arm64 ]
  RedundantVersion: version 4.15.2: slot(0) keywords are overshadowed by versions: 4.15.2-r1, 4.16.0-r3
  RedundantVersion: version 4.15.2-r1: slot(0) keywords are overshadowed by version: 4.16.0-r3
  PythonCompatUpdate: version 4.15.1-r2: PYTHON_COMPAT update available: python3_10
  DeadUrl: version 4.16.0-r3: SRC_URI: 404 Client Error: Not Found for url: https://dev.gentoo.org/~dlan/distfiles/xen-4.16.0-upstream-patches-2.tar.xz
  DeadUrl: version 4.16.0-r3: SRC_URI: 404 Client Error: Not Found for url: https://dev.gentoo.org/~dlan/distfiles/xen-gentoo-patches-24.tar.xz
Running repoman

RepoMan scours the neighborhood...
  ebuild.minorsyn               1
   app-emulation/xen-tools/xen-tools-4.16.0-r3.ebuild: line 425: Unquoted Variable

Note: use --include-dev (-d) to check dependencies for 'dev' profiles

RepoMan sez: "You're only giving me a partial QA payment?
              I'll take it this time, but I'm not happy."

[radhermit]

This probably isn't overly complex if tree-sitter support is leveraged.

For someone interested in working on it, I'd advise modeling it after MetadataVarCheck (without the decorator registry since this shouldn't need var-specific handling), but run across the entire parse tree instead of just for global vars.