Fix bug when requesting italic gfonts

[dewet22]

What does this implement/fix?

This fixes the missing left-hand side of the font spec. Currently an italic font will be requested as <family>:wght@1,700 which fails with HTTP 400 "Invalid selector: Too many values in tuple" referring to the right-hand side. The correct selector would be <family>:ital,wght@1,700.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Other

Related issue or feature (if applicable): n/a

Pull request in esphome-docs with documentation (if applicable): n/a

Test Environment

n/a, not hardware-specific.

• ESP32
• ESP32 IDF
• ESP8266
• RP2040
• BK72xx
• RTL87xx

Example entry for config.yaml:

 font:
   - file:
       type: "gfonts"
       family: "Archivo+Narrow"
       weight: 500
     id: font_weight
     size: 30
   - file:
       type: "gfonts"
       family: "Archivo+Narrow"
       weight: 700
       italic: true
     id: font_weight_italic
     size: 30
   - file:
       type: "gfonts"
       family: "Archivo+Narrow"
       italic: true
     id: font_italic
     size: 30
   - file:
       type: "gfonts"
       family: "Archivo+Narrow"
     id: font_default
     size: 30

Checklist:

• The code change is tested and works locally.
• Tests have been added to verify that the new code works (under tests/ folder).

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in esphome-docs.