Implement the LS APIs to support function definitions

[nipunayf]

Purpose

The PR introduces the following changes to the existing APIs:

1. Support function definition in the getNodeTemplate API
2. Support writing a function definition in the getSourceCode API
   1. For new functions, ensure "functions.bal" exists, as the LS will provide the text edit at the end of this line.
   2. For existing functions, the text edit will only modify the function name and signature

The PR introduces a new API endpoint flowDesignService/functionDefinition that retrieves function definitions using the components API response.

[NipunaRanasinghe]

Do we have test cases to validate if this new API works for function names with special characters like escaped/unicode characters (e,g, function names with spaces)?

[nipunayf]

Do we have test cases to validate if this new API works for function names with special characters like escaped/unicode characters (e,g, function names with spaces)?

Added

[NipunaRanasinghe]

LGTM