fix: don't add mode level if only one mode is present

[jonbrooks]

This PR is primarily for discussion. It addresses bugs I've been seeing with the most recent version of the plugin. I was seeing similar behavior as #271 , #273 and #274.

In my case, our token variables had a collection of primitives with a single mode, and then a colors collection with 2 modes whose values all referred back to primitive values. The desired output would look like so:

  "primitives": {
    "color": {
      "white": {
        "50": {
          "type": "color",
          "value": "#ffffffff",
          "blendMode": "normal"
        },
      "black": {
        "50": {
          "type": "color",
          "value": "#00000000",
          "blendMode": "normal"
        },
      ...
      }
    }
  "colors": {
    "dark": {
      "text": {
        "primary": {
          "type": "color",
          "value": "{primitives.color.white.50}"
        }
      }
    },
    "light": {
      "text": {
        "primary": {
          "type": "color",
          "value": "{primitives.color.black.60}"
        }
      }
    },

What came out was very confusing - with a "Value" level in primitives, and the colors had a mix of values grouped into a top level value object and others grouped into top level light and dark objects. I couldn't figure out any rhyme or reason to it.

The changes here disable adding the mode level if there's only one mode, and with this change, I get what I want in my tokens, but maybe there's a good reason not to do this - I'm not sure. I think it may have already been discussed around #258.

[coveralls]

Pull Request Test Coverage Report for Build 6357542902

• 0 of 3 (0.0%) changed or added relevant lines in 1 file are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.3%) to 66.2%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/utilities/getVariables.ts	0	3	0.0%

Files with Coverage Reduction	New Missed Lines	%
src/utilities/getVariables.ts	2	20.0%

Totals
Change from base Build 6296098097:	-0.3%
Covered Lines:	496
Relevant Lines:	733

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💛 - Coveralls

[lukasoppermann]

This looks interesting, I'll give it a try.