lean.lang: fix highlighting of identifiers

With the current Lean spec numbers inside identifiers such as a1 are erroneously highlighted as numbers and single quotes inside identifiers such as a' erroneously open a character literal. This PR fixes the highlighting of identifiers.

See also:

• https://leanprover.github.io/reference/lexical_structure.html#identifiers
• https://github.com/leanprover/vscode-lean/blob/master/syntaxes/lean.json

Additionally, I added the keyword partial, see https://leanprover.github.io/lean4/doc/functions.html.