Fix FORCE_COLOR to set exact level instead of minimum

Previously, FORCE_COLOR was used as a floor value and terminal
detection could return a higher level. For example, FORCE_COLOR=1
on a truecolor terminal would still give level 3.

Now when FORCE_COLOR is set to a specific level (1, 2, or 3), that
exact level is returned without further terminal capability checks.

Fixes #624

test/_force-color-fixture.js

@@ -0,0 +1,4 @@
+import {createSupportsColor} from '../source/vendor/supports-color/index.js';
+
+const result = createSupportsColor({isTTY: true});
+console.log(JSON.stringify(result));

test/force-color.js

@@ -0,0 +1,45 @@
+import {fileURLToPath} from 'node:url';
+import test from 'ava';
+import {execaNode} from 'execa';
+
+const fixturePath = fileURLToPath(new URL('_force-color-fixture.js', import.meta.url));
+
+test('FORCE_COLOR=0 disables color support', async t => {
+	const {stdout} = await execaNode(fixturePath, {
+		env: {FORCE_COLOR: '0', COLORTERM: 'truecolor'},
+	});
+	t.is(stdout, 'false');
+});
+
+test('FORCE_COLOR=1 gives exactly level 1 even with truecolor terminal', async t => {
+	const {stdout} = await execaNode(fixturePath, {
+		env: {FORCE_COLOR: '1', COLORTERM: 'truecolor'},
+	});
+	const result = JSON.parse(stdout);
+	t.is(result.level, 1);
+	t.true(result.hasBasic);
+	t.false(result.has256);
+	t.false(result.has16m);
+});
+
+test('FORCE_COLOR=2 gives exactly level 2 even with truecolor terminal', async t => {
+	const {stdout} = await execaNode(fixturePath, {
+		env: {FORCE_COLOR: '2', COLORTERM: 'truecolor'},
+	});
+	const result = JSON.parse(stdout);
+	t.is(result.level, 2);
+	t.true(result.hasBasic);
+	t.true(result.has256);
+	t.false(result.has16m);
+});
+
+test('FORCE_COLOR=3 gives exactly level 3', async t => {
+	const {stdout} = await execaNode(fixturePath, {
+		env: {FORCE_COLOR: '3'},
+	});
+	const result = JSON.parse(stdout);
+	t.is(result.level, 3);
+	t.true(result.hasBasic);
+	t.true(result.has256);
+	t.true(result.has16m);
+});