🎨 (basic): Organiza estilos de Basic e Intro

Divide `basic.css` en parciales SCSS bajo `assets/scss/basic/`, integra
`normalize.css` en `basic.min.css` y compila `intro.scss` por separado a
`intro.min.css`.

assets/scss/basic/_form.scss

@@ -0,0 +1,110 @@
+.form-required {
+    color: var(--val-color--danger);
+    margin-left: 0.25rem;
+}
+
+.form-label {
+    display: block;
+    margin-top: 1em;
+}
+
+.form-text {
+    font-size: 0.9375rem;
+    color: var(--val-color--text--muted);
+}
+
+.form-field > select.form-select,
+.form-field > input.form-control,
+.form-field > input.form-range,
+.form-field > textarea.form-control {
+    display: block;
+    padding: 0.25rem 0.5rem;
+    width: 100%;
+}
+
+fieldset {
+    margin: 2rem 0 1rem;
+    background: var(--val-color--bg);
+}
+fieldset > legend {
+    background-color: var(--val-color--bg);
+    border: 2px groove threedface;
+    padding: 0 0.5em;
+}
+
+.form-check {
+    display: flex;
+    align-items: center;
+    gap: 0.5rem;
+    min-height: 1.5rem;
+    margin-bottom: 0.25rem;
+}
+.form-check-inline {
+    display: inline-flex;
+    margin-right: 1rem;
+}
+.form-check-reverse {
+    flex-direction: row-reverse;
+    justify-content: flex-start;
+    text-align: right;
+}
+.form-check-input {
+    flex-shrink: 0;
+    cursor: pointer;
+}
+.form-check-label {
+    cursor: pointer;
+}
+.form-switch .form-check-input {
+    appearance: none;
+    -webkit-appearance: none;
+    width: 2em;
+    height: 1em;
+    background-color: var(--val-color--switch-off);
+    border-radius: 1em;
+    position: relative;
+    cursor: pointer;
+    transition: background-color .15s ease-in-out;
+}
+.form-switch .form-check-input::after {
+    content: "";
+    position: absolute;
+    width: 1em;
+    height: 1em;
+    background-color: white;
+    border-radius: 50%;
+    top: 0;
+    left: 0;
+    transition: transform .15s ease-in-out;
+    box-shadow: 0 0 0 1px rgba(0, 0, 0, .25);
+}
+.form-switch .form-check-input:checked {
+    background-color: var(--val-color--primary);
+}
+.form-switch .form-check-input:checked::after {
+    transform: translateX(1em);
+}
+
+input:disabled,
+input:disabled + label {
+    cursor: default !important;
+}
+input:disabled + label {
+    color: var(--val-color--text--muted);
+}
+
+.form-field > select.form-select,
+.form-field > input.form-control,
+.form-field > textarea.form-control {
+    border: 1px solid var(--val-color--border);
+    border-radius: 0.375rem;
+    background-color: #fff;
+    transition: border-color .15s ease-in-out, box-shadow .15s ease-in-out;
+}
+.form-field > select.form-select:focus,
+.form-field > input.form-control:focus,
+.form-field > textarea.form-control:focus {
+    outline: none;
+    border-color: var(--val-color--primary);
+    box-shadow: 0 0 0 0.2rem color-mix(in srgb, var(--val-color--primary) 25%, transparent);
+}