Improve snippets

[chemzqm]

• Insert nested snippets instead of merge snippet.
• Different python snippet use its own global values as expected.
• Accept all acceptable changes (not only the one change, one marker), unless the change affects snippet and other code.
• Avoid execute python when current session have snippet using python.

[fannheyward]

SnippetTextEdit is added in LSP 3.18, we also need to support this https://microsoft.github.io/language-server-protocol/specifications/lsp/3.18/specification/#snippetTextEdit

[chemzqm]

SnippetTextEdit is added in LSP 3.18, we also need to support this https://microsoft.github.io/language-server-protocol/specifications/lsp/3.18/specification/#snippetTextEdit

Need some work to allow snippet added to none active buffer.
What should happen when multiple snippets added to the same file?