Add a lineGap configuration option to the Font object

[sammyfeng0530]

Add a 'lineGap' configuration option to the Font object.

Description

Add a 'lineGap' configuration option to the Font object, so that the generated font file when calling toArrayBuffer includes the 'lineGap' property.

Motivation and Context

I need the 'lineGap' property to calculate the automatic height of text. Currently, the generated font file does not include this property, which causes inaccuracies in my text's automatic height calculation.

How Has This Been Tested?

const font = new Font({
    familyName: 'MyFont',
    styleName: 'Medium',
    unitsPerEm: 1000,
    ascender: 800,
    descender: 0,
    fsSelection: 42,
    lineGap: 500,
    tables: {os2: {achVendID: 'TEST'}},
    glyphs: glyphs
});
// It will be 500
console.log('font.lineGap :>> ', font.lineGap)

const buffer = font.toArrayBuffer()
const f = opentype.parse(buffer)
// It will be 500
console.log('f.lineGap :>> ', f.lineGap)
// It will be 500
console.log('f.tables.hhea.lineGap :>> ', f.tables.hhea.lineGap)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I did npm run test and all tests passed green (including code styling checks).
• I have added tests to cover my changes.
• My change requires a change to the documentation.
• I have updated the README accordingly.
• I have read the CONTRIBUTING document.