feat: add plot/vega/base/assert/is-signal-name

[Sachinn-64]

Resolves None.

Description

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This pull request:

• feat: add plot/vega/base/assert/is-signal-name

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{{TODO: add disclosure if applicable}}

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@stdlib-js/reviewers

[kgryte]

LGTM after clean-up.

[kgryte]

As the CI failure is expected, I'll go ahead and merge.