Add textDecorationColor, textDecorationWidth, and textDecorationStyle for Text

[rcoenen]

Three new properties for Text nodes, mirroring CSS text-decoration sub-properties:

textDecorationColor

Decoration color independent of text fill.

new Konva.Text({
  text: 'Selected text',
  fill: '#ffffff',
  textDecoration: 'underline',
  textDecorationColor: '#18A0FB', // blue underline, white text
});

textDecorationWidth

Override default line thickness (fontSize / 15).

text.textDecorationWidth(2); // 2px underline

textDecorationStyle

'solid' (default), 'dashed', 'dotted', 'double', or 'wavy'.

// Spell-check style red wavy underline
new Konva.Text({
  text: 'mispeled word',
  fill: '#000000',
  textDecoration: 'underline',
  textDecorationColor: '#ff0000',
  textDecorationStyle: 'wavy',
});

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

Changes

• Added three properties to TextConfig interface
• Both underline and line-through rendering use the new props
• Factory getter/setters with safe defaults (no behavior change for existing code)
• Fully backward compatible

Addresses #2037

[lavrton]

I think we should have all under the same property as in css. To have API minimal: textDecoration: 'underline overline #ff3028'. So no additional props are required.