VB6: Replace VB.NET's grammar by VBA's grammar

[DecimalTurn]

Description

Considering that :

• VBA and VB6 are closer in terms of syntax than VB6 and VB.NET. (As discussed in Visual Basic 6.0 or VB6 #5824 and Separate Visual Basic into VB.NET, VBA/VB6 and VBScript #4725).
• There is currently no open source grammar made specifically for VB6.
• The VBA syntax is more colorful (see comparison).

This PR replaces Visual Basic 6.0's current grammar (made for VB.NET) by the one made for VBA (which is already present in Linguist).

Checklist:

• I am changing the source of a syntax highlighting grammar
  • Old: https://github.com/peters-ben-0007/VBDotNetSyntax
  • New: https://github.com/serkonda7/vscode-vba

[lildude]

LGTM. Thanks.

Note: this PR will not be merged until close to when the next release is made. See here for more details.