Optimize input sequence in playback command

[tshino]

This PR is related to #176.

This PR adds optimization of the input sequence to the playback command.
For example, two consecutive '$type' commands will be concatenated into one and executed and recorded as one command.
This:

await vscode.commands.executeCommand(
    'kb-macro.playback',
    {
        sequence: [
            { command: '$type', args: { text: 'A' } },
            { command: '$type', args: { text: 'B' } }
        ]
    }
);

will be interpreted as:

await vscode.commands.executeCommand(
    'kb-macro.playback',
    {
        sequence: [
            { command: '$type', args: { text: 'AB' } }
        ]
    }
);

This optimization can improve playback performance, especially in a sequence containing a long typing.