Add support for 256 truecolor

Beforehand the `terminfo[colors]` and `COLORTERM` were not set properly.
The terminal already supports 256 truecolor, but cli tools might not use it, as the env variables were not set correctly.
This is fixed with this PR, the color is not set to `256` and `COLORTERM` is set to `truecolor`.

Fixes eclipse-theia#13523.

packages/terminal/src/node/shell-process.ts

@@ -61,15 +61,16 @@ export class ShellProcess extends TerminalProcess {
         @inject(ILogger) @named('terminal') logger: ILogger,
         @inject(EnvironmentUtils) environmentUtils: EnvironmentUtils,
     ) {
+        const env = { 'COLORTERM': 'truecolor' };
         super(<TerminalProcessOptions>{
             command: options.shell || ShellProcess.getShellExecutablePath(),
             args: options.args || ShellProcess.getShellExecutableArgs(),
             options: {
-                name: 'xterm-color',
+                name: 'xterm-256color',
                 cols: options.cols || ShellProcess.defaultCols,
                 rows: options.rows || ShellProcess.defaultRows,
                 cwd: getRootPath(options.rootURI),
-                env: options.strictEnv !== true ? environmentUtils.mergeProcessEnv(options.env) : options.env,
+                env: options.strictEnv !== true ? Object.assign(env, environmentUtils.mergeProcessEnv(options.env)) : Object.assign(env, options.env),
             },
             isPseudo: options.isPseudo,
         }, processManager, ringBuffer, logger);