[Suggestion] Do not try to guess a root if lsp--suggest-project-root failed

[ia0]

Currently, when lsp-auto-guess-root is set and lsp--suggest-project-root fails to find a project root (which actually never happens because of #3972), we fall back to lsp-find-session-folder.

This PR introduces lsp-guess-root-without-session to not fall back to lsp-find-session-folder and make lsp--suggest-project-root authoritative, thus giving full control to the user. Note that it rarely makes sense to set lsp-guess-root-without-session without setting lsp-auto-guess-root.

Alternatives considered:

• Change the behavior of lsp-auto-guess-root to always make it authoritative (previous version of this PR). This is a breaking change.
• Add a value to lsp-auto-guess-root, in addition to the existing nil and t (which semantics remain unchanged), to make lsp--suggest-project-root authoritative. Not clear what the value should be, and it could be considered a breaking change since lsp-auto-guess-root is documented as a boolean.

To motivate why the user may want full control on the definition of a project root, let's consider the following working tree:

foo/conf.y
foo/main.x
foo/bar/conf.y
foo/bar/lib.x

The projects of this working tree are:

• foo@x (directory foo and language x) contains foo/main.x
• foo/bar@x (directory foo/bar and language x) contains foo/bar/lib.x
• foo@y (directory foo and language y) contains foo/conf.y and foo/bar/conf.y

We can see here both the notion of nested projects (foo@x and foo/bar@x, see also #533) and overlapping projects (between language x and y).

With an empty session file and project/projectile not yet configured for language y, we can exhibit an inconsistency with the following operations:

• Open foo/bar/conf.y and fail to find a project root with project/projectile. Falling back to the session file also fails. The file is correctly not connected to an LSP server (because project/projectile is not yet configured).
• Close the file.
• Open foo/bar/lib.x. We find a root with project/projectile (because it's configured for language x) and use it, namely foo/bar. The file is correctly connected to an LSP server.
• Close the file.
• Open foo/bar/conf.y again. We fail again to find a project root using project/projectile. However, now the session file thinks that foo/bar is a valid root and will use it. The file will connect to an LSP server with the wrong root.