Adding new SCSS variables for letter-spacing

[simo026q]

Prerequisites

• I have searched for duplicate or closed feature requests
• I have read the contributing guidelines

Proposal

New SCSS Variables

$letter-spacing-base: normal !default; // Global default

$headings-letter-spacing: null !default; // Headings
$display-letter-spacing:  null !default; // Display sizes

$input-btn-letter-spacing: null !default; // Input and Button
$btn-letter-spacing: $input-btn-letter-spacing !default;
$input-letter-spacing: $input-btn-letter-spacing !default;
$form-select-letter-spacing: $input-letter-spacing !default;

These variables offer default values and null placeholders for customizable overrides, providing a flexible foundation for developers to adjust letter-spacing according to their design requirements.

Motivation and context

Some brands use tighter letter-spacing for headlines to create a sense of sophistication or to improve legibility at large sizes, which is not universally achievable with the current Bootstrap typography settings. Providing SCSS variables for letter-spacing makes it a lot easier to adapt Bootstrap more closely to brand standards.

[mdo]

Closed per #39866.