import * as t from "@babel/types";
import generate from "@babel/generator";
const param = t.tsTypeParameter(null, null, "T");

param.const = true; // HERE

const fn = t.arrowFunctionExpression([], t.blockStatement([]));
fn.typeParameters = t.tsTypeParameterDeclaration([
    param,
])

const p = t.file(t.program([
    t.variableDeclaration("const", [
        t.variableDeclarator(t.identifier("x"), fn)
    ])
]));

console.log(generate(p).code);

const x = <T,>() => {};

const x = <const T,>() => {};