chore(gatsby): migrate bootstrap load-themes to typescript

[nategiraudeau]

Description

Migrates packages/gatsby/src/bootstrap/load-themes to TypeScript.

Related Issues

Related to #21995

[gatsbot]

	Fails
🚫	Danger failed to run /app/peril-downloaded-gatsbyjs/gatsby@peril/rules/validate-yaml.ts.

Error TypeError

Cannot read property 'modified_files' of undefined
TypeError: Cannot read property 'modified_files' of undefined
    at Promise.all.Object.entries.map (/app/peril-downloaded-gatsbyjs/gatsby@peril/rules/validate-yaml.ts:149:25)
    at Array.map (<anonymous>)
    at Object.exports.validateYaml (/app/peril-downloaded-gatsbyjs/gatsby@peril/rules/validate-yaml.ts:148:52)
    at process._tickCallback (internal/process/next_tick.js:68:7)

Dangerfile

144|         twitter: Joi.string().regex(/^@/),
145|       })
146|     )
147|     .unique("id")
148| }
----------------------------^
149| 
150| const getStartersSchema = categories => {
151|   return Joi.array()
152|     .items(

Generated by 🚫 dangerJS