Fix(isLength): Correctly handle Unicode variation selectors

[koral--]

Previously, the validator would subtract all Unicode variation selectors (\uFE0F, \uFE0E) from a string's total length. This allowed for a string to contain a potentially infinite number of these characters while still passing a max length validation. For example, a string like "test\uFE0F\uFE0F\uFE0F..." could easily bypass a max: 4 check.

Variation selectors are now only discounted from the length if they directly follow a base character they are modifying. If they appear at the start of a string or in a sequence, they are correctly counted towards the total length.

References:

• Unicode variation selectors
• Variation_Selectors_(Unicode_block)

Checklist

• PR contains only changes related; no stray files, etc.
• README updated (where applicable)
• Tests written (where applicable)
• References provided in PR (where applicable)