Feature: textDecorationColor property for Text nodes

[rcoenen]

Currently textDecoration: "underline" draws the underline using the same color as fill. There's no way to set a different underline color, width, or style.

Use cases:

• Selection/hover highlights: blue underline while text retains its original color
• Spell/grammar check: red wavy underline on misspelled words
• Strikethrough pricing: colored line-through on old prices
• Rich text editors: full decoration control matching CSS capabilities

Proposed API (mirrors CSS text-decoration sub-properties):

const text = new Konva.Text({
  text: 'Hello world',
  fill: '#ffffff',
  textDecoration: 'underline',
  textDecorationColor: '#18A0FB',   // decoration color independent of fill
  textDecorationWidth: 2,           // override default thickness (fontSize / 15)
  textDecorationStyle: 'wavy',      // solid | dashed | dotted | double | wavy
});

When not set, all three fall back to existing behavior (no breaking changes).

Workaround: Render two stacked Text nodes — one behind with the desired underline color as fill + textDecoration: "underline", and the original on top. Works but doubles draw calls.

PR: #2038

[rcoenen]

PR #2038 now also includes textDecorationStyle (solid/dashed/dotted/double/wavy) and textDecorationWidth.

Additional use case: spell/grammar checking — red wavy underlines on misspelled words, which is a very common pattern in text editors:

new Konva.Text({
  text: 'mispeled word',
  fill: '#000000',
  textDecoration: 'underline',
  textDecorationColor: '#ff0000',
  textDecorationStyle: 'wavy',
  textDecorationWidth: 1,
});

This mirrors the CSS text-decoration sub-properties (text-decoration-color, text-decoration-style, text-decoration-thickness).