[FEATURE] Per-file VHDL standard override via inline comment directive/token

[nselvara]

Feature description

It would be useful to support a per-file VHDL standard override via an inline comment directive, similar to the existing -- vhdl_ls on / -- vhdl_ls off mechanism.

A common use case is writing wrapper files targeting VHDL-93 (e.g. for Xilinx IP that does not accept later standards) within a project that otherwise defaults to VHDL-2008 via vhdl_ls.toml. Rather than splitting the project into separate library entries with different standard settings, a lightweight directive at the top of the file would allow targeted overrides without any structural changes to the project configuration.

A possible syntax, consistent with the existing directive style:

-- vhdl_ls file_std="1993"

Motivation

Mixed-standard projects are common in FPGA development. Tool-generated wrapper files (e.g. from Xilinx Vivado) are often restricted to VHDL-93, while user-written RTL benefits from VHDL-2008 features. Managing this today requires either separate library declarations in vhdl_ls.toml or tolerating spurious diagnostics on legacy files.

Proposed behaviour

• The directive is recognised only in the first few lines of a file (similar to existing directives).
• It overrides the standard specified in vhdl_ls.toml for that file only.
• Both comment styles should be accepted for consistency: -- and --!.
• Valid values would mirror those already accepted by the standard key in vhdl_ls.toml.

Minimal reproducible example

-- vhdl_ls file_std="1993"

library ieee;
use ieee.std_logic_1164.all;

entity mre is
end entity mre;

architecture rtl of mre is
begin
    test: process
    begin
        wait;
    end process;
end architecture;

With the project standard set to "2008" in vhdl_ls.toml, the above file would be analysed as VHDL-93 without requiring a separate library entry.