Introduce a file icon for `.bal` files

[gayalkuruppu]

Description:
$Title

Describe your problem(s)
Currently, we do not provide a file icon for ballerina files by default.

Unless we install an 3rd party extension(ex: PKief.material-icon-theme).

Describe your solution(s)

Introduce an icon by default to support built in Seti file icon theme (vscode.vscode-theme-seti)